Manifold Vacuum Advance what am I not getting?

340inabbody said
Engine losses like 300 RPM min from P to in gear.

Engine should be tuned in gear with auto transmission. set carburetor first then timing, once timing is set fine tune carburetor.
I'd start with 18 initial, give or take. Probably end up 20 -24 degs initial
More likely the carburetor is the problem