acme,
Are the backfires through the carburetor, or out the tailpipe. With the timing light on the engine, the mark should only react to a change in RPM. You sound knowledgeable, so you know your mark is being moved, first by vacuum, then by mechanical.
I think I would take a look at the throttle plate in the carburetor and make sure it closes properly. If the timing was swinging way advanced with the burst of RPM's on a downshift, it could backfire through the carburetor.
The problem happens under high vacuum, closed throttle and no load!
You didn't mention if your rig has solid lifters.
Stuart
