Looking at Fred's Spring rate page

I came up with these 2 codes for my SB chev:

0-270lbs - GW (std spring) (moog PN# 5374)
0-310lbs - AB (HD F41 spring) (moog PN# 5450)

Will the HD springs sit higher than the STD ones, or will they just be stiffer? Is it possible to have F41 springs on a small block, or are those big block srpings?

The way the Chevelle suspension is designed, the rear should be pretty close to 1/4 the rate of the front spring so that they have the same rate at the wheel.

On the front, the wheel rate is 1/4 the spring rate.
On the rear, the wheel rate is equal to the spring rate.
The spring itself will have two specifications:
A) the spring rate in lbs/inch
B) free length

Your car will have a certain amount of weight on each wheel. After correcting for leverage affects, this will be the load rate. In a Chevelle the spring is about halfway up the LCA so it sees 2x the wheel load. So if there is 1000 lbs on the tire, there is about 2000 lb load rate on the spring.

If you want to control the ride height you need to figure out the fitted or loaded length. This will be the free length minus load rate / spring rate. So if we have a 500 lb/in spring it will compress 4" at a 2000 lb load (using our numbers from above). Thus if fitted length at proper ride height is 8" we would need a spring that is 12" free length.
Bounciness is a perception and can be caused by shocks *and* springs. The spring affects how much the wheel is allowed to move over a bump. The shock mostly affects what happens after that. The reason you see Civics hopping all over the place is that the suspension is not being allowed to move, much as would happen with a Chevelle with springs that are too stiff (or all poly in the rear).

Your combination is small block with less weight so small block recommendations are likely closer than big block recommendations. But I would question those numbers for SB. The way Chevelles are setup up front requires serious spring rates. Let's say, for discussion, that you have 2000 lbs on the front tires. That makes for 1000 lbs per tire. So to hold the car up there needs to be 1000 lbs pushing down on the tire.

But the spring is halfway up the LCA so rather than 1000 lbs it takes 2000 lbs. So if the fitted spring load is 2000 lbs it will take 8" of compression of a 250 lb/in spring. Now we put a big butt (or 2) in the car so the tire is supporting 1125 lbs, spring sees 2250 lbs, and that spring drops an inch.

But wait, the spring dropped an inch... and remember that the spring is halfway out the LCA so the wheel actually dropped 2"! That 250lb spring actually provided a rate of around 60 lbs/inch.

Welcome to the concept of "Motion Ratio" and "Wheel Rate". The MR takes into account the leverage between tire contact and spring and the WR is spring rate * MR * MR. WR is more useful because it tells you what is really happening

0-270lbs - GW (std spring) (moog PN# 5374)
0-310lbs - AB (HD F41 spring) (moog PN# 5450)

Will the HD springs sit higher than the STD ones, or will they just be stiffer ?

The ride height vertical dimensions frame to ground are listed in a chart. For the 1969 SS396 it shows different dimensions for options “Base” and “F40”.
With the SS396 “F40” option sitting about ˝ inch higher then the SS396 “Base” option.

According to my Moog catalog, the stock springs are part# 6326. Looking at the specs in the back of the Moog catalog, the #6326 look wimpy, .610" coil diamter and only 260lb/in spring rate.

The HD springs are part# 5380, these are .630" coil diameter and 318 lb/in spring rate.

To go a touch stiffer, Moog PN 5398 might be a good choice. They have a 362 lb/in spring rate. From what I can see in the specs, they might have your front end up about 1/4"-1/2" from the #6326
For comparison, my bigblock convertible with aluminum heads has 488 lb/in spring rate and it feels very firm, but not harsh. However, any firmer and it might feel harsh.

My hardtop, with a bigblock and aluminum heads and a fiberglass hood has 360 lb/in spring rate has a nice ride and has what I would describe as a medium firm feel.

I also have a 70 Cutlass (350) with 327 lb/in spring rate, and that is a little too soft.

Of course this is all subjective, but might help with your selection
The #5390 is a 336 lb/in spring rate and is really more for small blocks with AC.
The #5450 will have your car sitting about perfect to possibly up just a touch. And it has a 454 lb/in (I didn't make that up "454", maybe it's a sign) spring rate.
Or if you want it to sit just a touch lower than stock, get the #5536. It has a 488 lb/in spring rate

F41 suspension was about the rear stabilizer bar & h.d. shocks combined with standard rate springs.
As far as I know there were spring rates that were base equipment, F40 with higher deflection rates plus h.d. shocks (H.D. suspension) and F41 (added rear stabilizer bar plus h.d. shocks with standard rate springs (not h.d. suspension).
Springs with the base deflection rate were assigned to all different models as standard (that means F41 as well). Given the different body styles, equipment and engines, there were many different springs used to end up with that deflection rate.

F40 simply spec'd stiffer springs (e.g. 200 lb. vs. 150 lbs.to deflect) plus stiffer shocks.

F41 became available on V8 Chevelles in 1969 models (forget about the '65 Z16 for a minute). Only 722 cars were made with it in '69. F41 became standard on the Chevelle SS396 & 454 starting with the 1970 models. It was also available on non-SS models, but only if they had the turbo-Jet 400 (402). F41 remained standard on the SS models for '71, including the base 350 - 2 barrel SS. Again it was offered on non-SS models, but only with the 402. It included 14x6 wheels on those. No change for '72

I would like to know the free height, installed height, load rate, and spring rate, of the factory [1970 SS454 Chevelle F41 special performance suspension w/TH400 w/AC] front and rear springs ?????

[1970 Buildsheet code] [GC - front] [OQ - rear]

I would especially like to know the "correct installed height of the springs", front and rear of a 1970 Chevelle coupe ??

I would like to know the free height, installed height, load rate, and spring rate, of the factory
[1970 SS454 Chevelle F41 special performance suspension w/TH400 w/AC] front and rear springs ??

[1970 Buildsheet code] [GC - front] [OQ - rear]
Build sheets I have of 396/454 Chevelles with A/C generally show GC coded front springs.
Specs from GM show them to be part #3960686:
Cut-Off Length - 125.94
Wire Diam, - .659
Total coils - 9.22
Deflection rate - 435 lbs per inch
Free height - 15.32"
Working height -11.7" @ 1780 lbs

Ride heights for a 2-door sport coupe is shown as 14.3" front bumper-to-ground, 15.1" rear bumper-to-ground,
27.4" headlamp-to-ground. Sadly it doesn't show where the measurements are taken from on the bumper but I'd imagine the centerline on the headlamps.

The Factory Assembly Instruction Manual may be a better source.
UPC 0, sheet A11 shows measuring 31.5" rearward of the centerline of the front wheel hub, body (rocker panel) to ground clearance is shown as 9.70" +/- 0.38" for SS coupe & conv.
Measuring 24.6" forward of the rear axle centerline, the measurement rocker panel to ground is 9.46" +/- 0.38".
The following is from KC cars: I got this info from the GM restoration kit.
Boxes 13 & 14 ~ Front/Rear Springs: This happens to be the spring for the F41 in a L34 396 sport coupe.

example: GB for a front spring is part #3960665 with a cutoff length of 125.89, wire dia. of 659, total coils of 9.22, deflection rate of 435 lbs per inch, 15.52" free height and 11.7" @ 1650lbs working height.
Examining numerous examples shows what springs were installed with what other types of options.

The same F41 option in a LS5 454 sport coupe got springs coded GC which differ only in cutoff length (125.94), 15.32" free height and working height at 11.7" @ 1780 lbs.
F41 suspension was not a single, all-inclusive set of springs, shocks, but tailored for the specific engine/body/etc.
The numbers/letters in the build sheet blocks are the 'broadcast' codes for the the specific item and many of the items had this code either ink stamped, color coded, or paper tagged on them. Suspension parts like springs come to mind as items with paper tags you see all the time on restored cars while other things like tie rod ends, drag links had a daub of color, all *shortcuts* (if you will) for the workers installing those parts as they had to be identified by some means ... GM part numbers were too long for the installers to have to find. A spring with [GB] on the tag would naturally be easier to find, than looking for a tag with the part number on it.

Examples on 3 Baltimore cars.
1) - L34 396 coupe w/M21 transmission.
2) - LS5 454 coupe w/M22 transmission.
3) - L78 396 convertlble w/M21 transmission.
*)All three have the same (GB coded) front springs,
*)the LS5 coupe and L78 convertible have the same (QQ coded) rear springs, the L34 coupe has (OO) rear springs.
*)None have A/C and you'd think the front springs may differ but not so, it's the rears.
*)All use the same front and rear shocks.
It appears from previous posts, [1970 Buildsheet code] rear springs coded [OQ are 12.19" free height] and the
[OO coded springs are 11.94" free height] but both sets have a deflection rate of 160 lbs per inch ...