Backspacing question

Unless you've got 1/2" offset springs you're not going to get a 10.5" wide tire in there. Not gonna happen. You need at least a 1/2" to the quarter minimum, and about that much to the spring unless you're running larger diameter rims. So, with most Darts having about 11" from quarter to stock location springs it's not gonna work.
Show him my old pics
;)