I run 17x9" rims with 275/40/17's all the way around, using a 5" backspace. Now that I'm running a little more negative camber, I've got no rubbing issues anywhere. Prior to cranking up my camber to -0.9, I was rubbing a little on the right front fender lip.
Here's the thing though. 18" rims mean that the tie rod end actually fits inside the rim, so you can run more backspace because the tie rod end is usually the primary limiting factor. With the 17's I was pretty much limited to 5", MAYBE 5.125", which would have been better, but would put me right on the tie rod ends.
Still, with a 9" rim I would say around 5.25" of backspace would be ideal, more than that and you might start to get issues with tires rubbing on the frame at full lock. Depending on tire size of course, but it looks like the FR500's are mostly 9" wide, so 255-275 is where you'd be anyway. Check with Dr. Diff, those brakes may increase your track width a bit, I've heard of some adding as much as a 1/4" per side. That's actually a good thing, since it would let you run an additional 1/4" of backspace.
The center bore can be smaller on the true mustang rims, most of the aftermarket rims have larger bores. And if you're running aftermarket brakes that may not be a concern anyway. Since the FR500's I've seen are in the 6" backspace range, you could probably just run a set of hubcentric spacers anyway, most of those are 1/4" to 3/8" wide, which would probably get you in the ballpark with a 6" backspace rim and a 1/4" wider track with the aftermarket brakes.
In the rear it depends on whether or not you're stock. As I mentioned, I run the same 17x9's with 5" backspace in the back with a TON of room for bigger. But I've got 1" offset spring hangers and a B body rear. If you've got the stock E-body rear and stock width springs, you won't be able to go to a 9" rim with much more than 5" of backspace without spacers. A 1" offset kit would probably clear you for larger though, and with the longer E body rear that 1" of space per side could go directly into the additional backspacing- which again should out you in the ballpark for the FR500's.
