Try 8-12 btdc initial.-22 deg.advance starting in at 2500rpm all in by about 3500rpm. Fuel grade,driving style,carb/ injector tuning will need to be taken into account also,so these numbers may need to be plus or minus to avoid detonation,and overheating. Use vacuum advance canister thet came with the engine. there's more, but it will get you started.Sparky
Any time the timing is in question I double check the timing mark. Aftermarket timing tabs can get mixed up (Chevy's moved them more than once) plus there's always a possibility that you could have spun the outter ring on your harmonic making the mark useless. Five minutes of checking might save you a world of grief. Next verify your advances are working - disconnect the vacuum advance (***uming you have one) and make sure it actually moves!!! next rev teh motor - does the mechanical advance move????? If so how much?? Once you know everything is working and your starting point is right THEN you can fiddle with a number that actually makes some sense.