I'm thinking that you need a new distributor. It seems that once the vacuum advance is pulling the plate it's throwing the pickup out of phase. You can send your distrib to FBO and have them set it up ($70 iirc) or buy a new aftermarket one. You'll most likely have the same problem if you change out to a auto parts store rebuilt. Those vacuum advance plates get super wobbly with age. They weren't all that hot new either, lol!
Yeah, it's seems to be a big carb, but by your description of the problem, it sure doesn't seem carb related. I've a got a 650 DP on a 5200+ lb truck with a super tight converter 3.55 gears and 33" tall tires and seems to want even more.