Vac advance should be attached to the ported vac port, not straight intake manifold vac, from the factory (PLUS a possibly deteriorated factory vac hose map, underhood). Reason is that ported vac lasts longer than intake manifold vac when you throttle into it. With full intake manifold vac, when you open the throttle quickly, no vac advance immediately. UNLESS . . . you get some of the vac delay valves which Ford was famous for using in the 1970s, put it inline (even two in parallel as GM did on a "factory fix") and turn them around backwards so it maintains vac in the vac advance can . . . which can cover for a very lazy dist mechanical advance curve.
Glad that the car is runing better.
Just some observations,
CBODY67