Manifold Vacuum Advance what am I not getting?

340,
You have let yourself be brainwashed by Turk & his nonsense. For the other MVA non-believers, we should say a prayer for them & hope they have a quick recovery......
These people do not understand the combustion process & WHY more timing is needed.
Thousands, if not millions of GM cars, came with MVA. Below is a page out of the 1967 Pontiac manual stating the use of MVA. These cars had 6* initial & 20* MVA added so that they idled with 26*. Small cam, 197 @ 050, 10.75:1 CR.

Your 42* sounds right & do not let some dumb a**e talk you out of it. My own car has 48* at idle, has had it for the last 15 yrs. Another car I tune for drag racing 3950 lb without driver, 3.31 diff, street driven & does 1000 mile return trips, has run 11.65 @ 119 mph. No tuning at the track & the driver was new to drag racing. It idles with 48* via MVA @ 900 rpm in gear.

I did a final tune recently on a 440 I built. TF 240 heads, 10.X:1 CR, Mopar M1 4500 intake, TQ carb. It idles with 42*. I think stunned would be the right word....The owner was stunned at the improvement in idle quality, vacuum etc between 12* at idle & 42*...