to the guys that know everything about dart swingers...

That's an 8.75 rear end. Score! (or a 9" - LOL).

Torq thrusts depends on bolt pattern. Hopefully dude upgraded to the 5*4.5 when he was doing everything else. That would be a big plus.

With those wheel wells you should be able to run a 15*8, but you're not going to be able to have more than 4" or so of backspace (depending on brakes), unless you use a spacer.

Springs don't appear to have been relocated, but obviously, that's a guess.

My car pictured above runs a 15*8 with 4.5" backspace and a 3/8 spacer. I am using B-Body brakes, whichever was the smaller of the two options (10*2.5?).