Large RPM and Vacuum drop when shifting from park to gear

I use my tongue to block off the hose in between sucks. Yes, I know how that sounds. :rolleyes:

I'd like a vacuum can that is fully advanced at 8" Hg. since that is all my engine can muster at 1200 rpm idle. I think FBO (Don) can make one, but I don't think there is an off-the-shelf unit that will fit my Mopar distributor.
As it is, I run 16 initial, 27 at idle, long slow centrifugal advance thereafter, plus a vac can that starts advancing at 10" or so. Cruise is 14-15" which is full advance.