manifold vaccum for vaccum advance ?'s

Just a thought, have you stuck the stock MP distributor in there?
They are adjustable with an allen key thru vaccum port.
Smoking while letting off is a sign of valve guide troubles.
I would do a compression test and leak down test before doing much more work.

Mopar has not made that crate motor for alot of years, I just tore down a machine shop fresh 340 that sat so long, the oil was dry on the piston skirts.