Setting timing for /6 with new electronic distributor

You can buy advance cans with different amounts of advance built in.
Or you can mod a good-working stocker by grinding on the stops. The most I could get from mine was 22*. I think member @TrailBeast got 24 out of his. Ours are both SBMs.

Timing for a manual trans will be different than for an automatic. This is because a manual will spend a lot of time at rpms lower than the stall rpm.
Altitude will also play a role, because as the engine goes ever higher, the power will decline, and the throttle will have to be continuously opened further, to maintain rate of climb or road-speed......... because the EFFECTIVE compression ratio is continuously dropping lower.
The power is declining on three fronts; lack of air, surplus of fuel, and declining timing with declining rpm. There is no cure for the destruction of the A/F ratio besides a computerized fuel injection system, and no cure for the loss of power besides supercharging, or a bigger engine.