how critical is it to set the rpms at the specified level before checking or setting the timing on a smallblock chevy? how accurate would it be to check timing with the car at normal idle in park?

Setting the timing at idle will be as accurate as setting it as any other speed, it may not be right but it will be as accurate.

thanks!

Depends on what you are looking at for total timing. Setting at idle does not take into consideration the mechanical advance in the distributor.
Setting the timing at a particular RPM still doesn't take into account the mechanical advance in the distributor.
The mech advance is RPM dependant.
So what you need to know is approx. where the engine "likes" the total timing to be. Is it 36 degrees BTDC? Is it 42 BTDC? Or is is it less or more.
Te set this you need to know how the "curve" in the distribuor is set up.
Stock distributors often times don't achieve total mech advance until 5000+ RPM. Not the best for performance.
You can modify the stock distributors to achieve total advance earlier by changing the weights/springs. Kits are available to do this. It isn't cut and dry.
There are other things to consider. Like how much advance will the distributor allow? This varies depending on what application the stock distributor was used for.
Vacuum advance also figures into the equation.
Perhaps Todd DeLamuca will chime in here, he knows alot more than I do about this subject!
Ron

thanks ron,its a stock rebuilt 305 or 350 in a 1985 silverado with what appears to be the oem distributor.the truck originally had a 305.i bought the truck for 250.00.the catch is the engine has been replaced before,the truck has been sitting mostly since 2002, and the previous owner is old and cant offer any info on the engine.i'm gonna try to get the block casting number off the back of the block tomorrow.i cant find one on the sides of the block.i'm currently replacing all the cracked vacuum lines and fixing all the little problems that appear after a vehicle has set up too long.all i can say is at least it runs!

oh yeah,i bought it to haul my chevelle to the drags when everything is finally finished.

Then I'd set it at idle with the vacuum disconnected. Start at maybe 10 BTDC, if it pings, retard it till it quits. If it doesn't, advance it till it does.
Tough to call when you don't know what the dist is set for, and when you don't know what the engine wants.
Hows the Skylark?
Ron

to extend what ron said, lets say your dist has 10 degs in it-thats 20 degs @ the crank, add, lets say 12 degs @ idle, thats 32 degs total @ whatever rpm it reaches total adv.-now lets say the dist has 15 degs, thats 30 @ crank, if you add that same 12 degs @ idel, now you see 42 degs total-on a perf. engine, thats a lot of diff-hope that kinda sheds some light on how it could be a prob., but sounds like what your doing should'nt be a problem-follow what ron said, you'll be ok

thanks guys,i hope to have another go at it tomorrow.