belvedere rear end

That '67 B-body rear end is going to be pretty wide for a Dart. They work well on Dusters/Demons/Dart Sports, but they have more room out to the quarter. You'll need rear wheels with about an inch more backspace than "normal". Not sure what you're running back there, but for an 8" wide rim that would about a 6" backspace. For a 7" wide rim it would be about 5.5".

Also, about the /6 driveshaft. Some were tapered down at both ends, which makes shortening them difficult/impossible. Not sure if that's the case with yours, and or course it depends on how much the driveshaft is being shortened.