Timed or manifold vacuum advance?
heres the deal...
You want as much initial timing as it will start with 'fully warmed up'
Test drive trying different total/full advance setting till the best performance is achieved without detonation/ping
find a way to have both, be it by welding up the slots in the distributor or a bushing in your msd distributor [mine is custom]
Now set the rate of advance to suit the power band & or convertor stall [if equipt]
Basically have it full advance as soon as it can with out ping/detonation
Thats where the performance comes from.
If you want to get better mph during cruise [not acceleration] run the vacuum advance can.
If the car run cooler with the vac advance it might be because you don;t have enough total to begin with.
OK, now THAT was sayin somethin. And even I, "the guy with no brain" can understand that.
The way I used to do so is essentially the same:
I start with TOTAL and adjust total for max perceived HP (never had dyno, had to use 1/4mi) and use lightweight or no springs to ensure full advance.
So you check that with your light at high RPM, and winder 'er up to be sure that it's not got any more advance left, or spark scatter
Then use stronger springs to get down to initial and check that about as slow as she'll run, and adjust slots so that initial is where ya want with the previous total still in place.
Then adjust springs for best midrange/ drivability.
And last screw with or eliminate the vacuum advance. Frankly (mileage) some of the time it didn't seem to make much difference, probably because of how hard we drove them.
(If you are into the pedal a lot, the vac advance isn't gonna do squat)
Back when "we had gas" I used to be able to run as much as 38* and sometimes 40* total on about a 20-22* curve, so initial would be 16-18*