If the distributor was off a tooth, I don't see how it's possible that it could get up to even 45 mph.
IIRC the dizzy has ten teeth. Being one off would put the dist 36 degrees off, and given that the rotating assembly goes twice as fast, that would put it 72 degrees off....
I had one where the timing chain was off a tooth, and that would cause only a 10 degree difference. It barely even idled, and would choke out when you gave it gas... unloaded.
Seeing that it's drivable up to that speed, I don't think it's the dizzy.