The HEI usally will advance about 24-28 degrees in mechanical.
I would say your initial (idle at 850 ish) will be in the 10-12 degree mark for a warmed over performance motor. Then while you have the motor reving at 3000 rpms you will need to see what the timing is at - this will be your total advance. this should not go over 38 for your Pontiac and probably would be better at 36ish.
the advanced timing light work off the balancer mark lining up with the zero mark on your timing tab on the timing chain cover.
Remember ALWAYS check timing with vacuum advance disconnected and plugged on the carb or manifold vacuum side.
When you hook the vacuum advance back up it WILL make the timing go up and idle too. Do not FREAK this is normal. Just adjust the idle back down a bit and see how it goes after that.
