Pre purchase opinion on these tire and wheels used please

I would bet large the rears with 3 1/2 b.s. will NOT work on your dart.
I have a couple pairs of vintage 15x8 1/2 wheels with similar backspace. They don't fit anything that doesn't have a full open radius wheelwell opening (like a 70s mustang.)
Depending on how your 8 3/4 is configured, you probably need a a 4 1/2-5" bs for the rear. But a tape measure is your friend.
Edit: a 15x7 is much easier to fit than a 15x8 or 8 1/2.