Tire/Wheel choice

Either of those rims should keep the wheels inside the quarters. The 15x7's with 4.25" of backspace will put the centerline of the rim a 1/4" further inboard than the 15x8's with 4.5" of backspace.

With a Dart Sport, an 8.25" rear axle and the stock spring locations you should be able to fit a 275/60/15 inside the quarter, and that should work on the 15x8 with 4.5" of backspace. The 8.25" rear axle is narrower than an A-body 8 3/4 with BBP axles, so actually the 15x8 with 4.5" of backspace should work fine. A 255/60/15 on a 15x7 with 4.25" of backspace should clear pretty easily.

A 295 won't fit inside the quarters though. Not with stock spring locations and untrimmed quarters. Only way to make that work would be to hang them outside of the quarters. I have 295's on my Duster, but I have a 1/2" spring offset and another 1/2" taken off of the quarter lip to make that work.