The engine responds the same whether the vacuum line is connected to the ported or manifold vacuum source.
It also does the same with 3 different carburetors including a sweet running Edelbrock 750 that allows an almost perfect idle.
What is not being changed? The distributor. I would take a HARD look at the vacuum advance canister. EASY PEASY to diagnose. BUT YOU NEED A VACUUM GAUGE LIKE I SAID IN MY FIRST POST, DAMMIT.