What's your initial spark advance? The more, the merrier, if your distributor has a curve that will tolerate that much once you're on the road. That usually means a custom curved distributor.

My old Dart's 440 was also low-compression, and I was running a MP 284/484 beast, with a TIGHT 11" converter. Yeah, not a great combo. Idle vacuum in gear was about 8", in neutral was about 11". Unfortunately, I was a po' collige kid and couldn't ever afford to get my distributor curved - and didn't know how to do it myself. So, 12* was about all I could manage and still drive the car at highway speeds. If I could have gotten 18* it would have been a lot happier.

Clair