Manifold Vacuum Advance what am I not getting?

Nope, MVA is not a crutch for a poor build. MVA is but one method of giving the engine the timing it NEEDS.

OP has found that HIS engine has best/smoothest idle with 42*.
THAT IS NOT BECAUSE IT WAS BUILT POORLY OR THE ENGINE BUILDER DIDN'T KNOW WHAT HE WAS DOING. IT WANTED THAT TIMING BECAUSE IT IS MOST EFFICIENT WITH THAT MUCH TIMING.

What about racers who lock their dist? They are giving the engine the timing it NEEDS, does the same as MVA. Are they ALL 'piss poor' engine builders...... Yes those ones, winning races with locked timing????? Of course not.

People who say or think otherwise do not understand the dynamics of combustion.

Stock LS engines have about 27-28* @ WOT. They have a very efficient chamber that does not need a lot of timing. In stock form, they STILL idle at 22*. Not 5* or 0*.
Because that is where the most efficient use of the fuel is made.
Extrapolate the #s for our old bangers that need 36-38* @ WOT & SUDDENLY 30* at idle for stock or near stock doesn't sound so bad....
Engine might need 45* at cruise. So what's the difference between idle timing & cruise timing ? Uhhhh, nothing.......
In both instances, the air is throttled. The air is less dense & needs more time to burn, hence more timing needed.