How much backspacing?

I just installed a set of 15x10 Welds with 5.5 backspacing on my 86 GT. With 295/50/15's the tires are going to definitely rub on the outside and will be real close on the inside. I don't run the quad shocks and I have removed the brackets so they won't be a problem. I plan on mini tubbing and narrowing the rearend slightly to get the clearance I need. I have seen Mustangs with wheels that have 6.5" backspacing, but no 7" yet. You will want to do some careful measuring before purchasing the wheels and I'd bet you'll have to massage the wheelwell with a hammer to get enough clearance.
Good Luck.

I have 15x10's with 6.5" BS and it's a tight fit. I would guess IF you can fit a 7" BS wheel on there, you're talking clearance of a piece of paper or so. What you're basically doing is taking the whole width of the stock rim and putting it behind the face of the drum. That's a LOT of backspacing.

As for ore backspacing with 4-lug wheels -- that's BS. Maybe for that company, but not in general. You could put 16 holes in the wheel and still have the same backspacing and whatnot.