1/2 ton rear to 64 Dart?

Depends on the truck, but too wide regardless.

Early Dakota's (pre '97), some of which also came with 8 1/4's, had a track width of ~61". The first gen Barracuda's have a track width of 55.6". So, about 3" too long per side.

You MIGHT be able to make that up if you're planning on running modern rims, which typically have a ton more backspace. But even with the larger backspace 17's or 18's I've seen you'd be right at the limit. With stock offset rims you'd be into the fenders and then some.