Manifold Vacuum Advance what am I not getting?

First thing you need to do is run a compression test and post those numbers.

Vacuum advance cans generally pull in about 20 degrees. So time the thing at 20 initial at the distributor, limit the total mechanical plus initial to about 34. Then plug the vacuum can into manifold vacuum. This will give you about 40 degrees initial at idle and part throttle, but the extra the vacuum can pulls in will fall off on acceleration. As mentioned, get an adjustable vacuum can and turn the adjustment screw all the way clockwise to start with. Stop trying to frikkin understand and just do it. It'll either work for you or not. If it doesn't just use ported vacuum.