I'm in the same boat as you, have stock spring rate and at times wish they were a little beefier. Car has settled a little more with full interior and miles.

I know this isn't the best solution, but it might help. This is a link to Moog's spring filter. You can view springs by their specifications and it helps to see what's available. Pretty much a crap shoot, was tempted to start experimenting and putting data on here, but after the price of everything went up, I shelved the idea.

Moog Coil Springs | Moog Suspension Parts

This Catalog of Moog Coil Springs lists 9 dimensions for each coil spring to help you find the right suspension springs for your vehicle. Whether you're replacing sagging springs or working on a project vehicle, find what you need here!

www.moog-suspension-parts.com

For instance, I'm running Moog 6200 springs up front. Put 6200 into the part #. It shows you all the specs. Then to get viable options (theoretically) I take and put 11 in the installed height, 3.6 for inner diameter, then delete 6200. Boom, full list of coils that are potential candidates. You can click the arrows next to the headers at the top of the list and it will be sorted by that header.

It really shows that the major component of rate is the coil size. The 6200 are rated at 290 and have a 0.62" coil. I've been eyeballing the 5400 and 6041, figure they would compare to aftermarket springs.. Just wish the price would come down so I could mess with them. Yes, would have to spend some time seating them then cutting, but it'd be worth it.

 

I have Umi tubular upper and lower control arms and drop spindles on my 66. I’m running .5 tall bj on the uppers and stock height on the lowers. I tried using their springs 500lb spring rate. After numerous attempts trying to get the desired ride height I ended up biting the bullet and bought Viking coilovers. Love the ability to change ride height and shock adjustments in the matter of minutes. Now I have a pair new KYB gas adjust shocks, UMI springs(cut 1/2 coil) and new TRW 6200 spring sitting around for just in case.

 

How much were the coilovers? What spring rate?

 

Al,
I think I paid around $550.00ish. The spring rate is 550 lbs.Double adjustable. There are several quality dealers, depends on how deep your pockets are. Here is a pic during the install. I don’t have current pic of El Camino. I have my front end sitting at 24 1/2” at center of fender. 225x50x17 tires and wheels.

 

Do you have a BB? Hows it ride with 550's in there?

 

UMI 1” drop plus your tall lower ball joints should get you pretty close to the height you already have.

That way you could trim if needed.

The other option would be a 2” drop from UMI. Then you could add spacers if needed. (Which I think you would)

 

2" UMI drop springs front and rear. Monroe Sensa-Trac shocks. Stock control arms and full iron 396.

 

I have a 69 el Camino SS396 454 th-400. It has BMR 1" lower on the front and BMR 2" lower in the rear. Its perfect! el Caminos don't look good and handle right with the rear end jacked up. I've found having the front higher then the rear, really makes things better.