need help purchasing street radials for my 74 dart sport

What rear axle do you have? Not all A-body rear axles are the same width, they differ by model. 7.25/8.25” BBP rears are narrower than A-body 8 3/4’s with BBP axles. I assume you have the 5x4.5”, BBP.

275’s will fit most Dusters/Demons and Dart Sports with the proper backspace.

In the front it will be close to the fender with 215’s. On a 15x7” a 4.25” backspace is better up front. Again, that’s assuming you have the factory BBP disks.