72 dart rear tire size

Well, if you have 255s on it now, it's easy to find out. A 275 is just under an inch wider in section, so add a 1/2" on either side, and see if you have the room. If it's too close to the spring, but has room to the fender, there is always the wheel spacer option (billet only, please).
The 255/70s might be harder to fit. They are tall enough that they might not clear the front edge of the wheelwell opening.
(I have 255/70s on my b-body..... but the whole rearend is moved back an inch.)
Edit: check for inner wheel tub clearance too, at the top, not just space to the springs.