The E-body's are really pretty easy for tire sizing. No reason at all you can't run 17's or 18's on factory suspension. I have a set of Hotchkis UCA's on my car, but everything else is pretty much stock. For the most part, all you need is a zero offset rim. If you're planning on going crazy big, a little more backspace will help, especially with 18's because you don't have to worry about rim to outer tie rod clearance.
I have 17x9's with 5" backspace (0 offset) on my Challenger with 275/40/17's. I needed to do a little adjusting on the passenger fender, but it clears now with a little negative camber on the front tires. But a 275 is darned big up front. I would run a little more backspace, but with the 17" rim I still have to worry about the outer tie rod. On my car I could get away with a 17x9" with about 5 1/8" backspace, but I don't think I would clear 5.25".
With an 18" the outer tie rod actually fits inside the diameter of the rim, so you can run as much backspace as you want as far as the tie rod is concerned. But, on my car I don't have much tire to frame clearance at full lock, so I would say that with a 9" rim 5.25" of backspace is probably about as much as you can run anyway.
For an 18x8" I would say you'd want between 4.5 and 5" of backspace, and that should clear any tire at or under 26" tall that you can fit on an 8" rim (shouldn't be bigger than 255 wide). For an 18x9" rim you'd want 5" to 5.25" of backspace. With a 275/35/18 you'd be tight on the fenders with 5" of backspace (0 offset), but anything narrower than that should be fine. More than 5.25" of backspace will probably cause some frame rubs at full lock, but keep in mind you can always add a small spacer if you go with too much backspace, nothing you can do for too little. Personally, if I was looking for another set of rims for my Challenger I'd be looking for either 18x9.5's with 5.5" backspace or 18x10's with about 5.7". But the wider rims like that may not clear the stock UCA's at full lock and full droop, I know the 9" wide rims do.
Also, final note, all of the backspacing numbers I list above are for stock 73+ disk brakes, either the stock 10.98's or the larger 11.75's. I run the 11.75 mopar disks. I say that because the drums have a narrower track width, and because aftermarket disks usually increase the track width. If you're going aftermarket disks you have to find out how much the change the track and adjust accordingly. And there's a decent amount of body tolerance with these cars, so the more you push the limits with larger tires the more likely you are to have to make some adjustments, not everything is always perfectly centered on these cars. Mine isn't, I had some minor rubs on the passenger fender and rolled the lip, but I never hit the driver's fender. Go figure.
I use this website,
http://www.rimsntires.com/specspro.jsp to do wheel/tire combo comparisons. Once someone has a combo that fits, you can enter in what you're looking at and see how it measures up for clearance. So, you can enter my combo, which pretty much has no room to the fenders and just a little to the frame, and see how your combo stacks up.
Anyway, my '72 Challenger with 17x9's, 5" backspace and 275/40/17's. Wheel are the same size front and rear.
