RE: chevy 350 ci ignition timing
Depends on the cam, compression ratio, etc.
Start with a setting of 10deg btdc at idle, vac advance disconnected and plugged for a "safe option". Reconnect when done.
Advance slightly in 1 degree steps and test-drive the car.
If you encounter pinking, back off the advance by 2 degrees.
If you have built up a "standard package" of performance parts (eg Edelbrock Performer RPM package), the supplier will be able to advise you.
However with a "mix and match" the only way to get an exact figure for YOUR engine, would be to have it dyno'd - and tweak the initial, mechanical and vacuum advance to what the engine needs.
GD427 Euro, 350 Chevy, Tremec 3550
MSD, Megasquirt Fuel Injection (TBI)