Just a couple things.
I don't trust timing marks on stock Chevy dampers, or the timing indicators on the timing cover either. I have seen way too many outer rings "walk" with any amount of high RPM operation. This is about the only reason I like Fluidamper and other 1 piece units. If you have the time, find the true TDC and mark your inner and outer pieces of the damper accordingly.
If you have access to a good advance timing light, set your total advance to 36deg with everything fully advanced, set it and drive it. Advance or retard from that point to suite yourself. Don't worry about your initial advance because in the big picture, initial advance doesn't matter nearly as much as the advance numbers when your engine is at happy RPM. Like mechtech said, this is a fixable problem and you have to know the specs/info you are dealing with to get an answer.
Verify the distributor advance is working smoothly, and is hooked up to the proper port on the carb. You would be surprised how many hook up the dist to manifold vac instead of venturi vac. Verify the advance weights arent sticking. I ran into one HEI that was so worn the advance weights would stick and cause a 1600 RPM idle at times, that was fun to track down.
If your timing is going "bouncy-bouncy" all over the place either it is computer controlled and acting normally, or you have a problem. Slopped out timing chains, and worn distributor shafts/housings are the most common causes. Really mean roller cams and even a messed up oil pump can cause spark scatter, but it is a cold day in **** when you run into that in the real world.
