back spacing

The tire will make a big difference, too. You might want to pick your tire size first, and compare its diameter and tread width with what you have now. Here's a link to a handy calculator, that also lets you play with rim width and offset:
http://www.rims-n-tires.com/rt_specs.jsp

But offhand I'm guessing a 15 x 6 would be no problem, 15 x 7 or up you might need to be right on the money with the backspacing. On Barracudas the critical point is the lower front of the opening, but I don't know about Darts.

Backspacing amount depends on total width. The idea is you want the mounting surface pretty much smack in the middle of the rim, or a little to the outside. So if you have a 7in rim, you want 3.5 to 4in backspacing, or something like that. Hope that helps.