I have 17x9's with 5" of backspace on all 4 corners of my '72 Challenger with 275/40/17's. In the front 18x9.5's should be right on at 5.5" of backspace. 275 is about as wide as you can go, I had to do a little rolling on my passenger fender to keep from rubbing, and that's with -.9* of camber. I have a little room to the frame at full lock, but it isn't much, probably a little less than 1/2". But in the front, 18x9.5's with 5.5" backspace and anything up to a 275/35/18 should fit.
In the back I have a B body rear end with Dr Diff's 1" spring relocation. With the 275's and 17x9's I have exactly 1.5" to the springs and the quarters, so a 12" rim is going to be pretty tight. You should be able to run an 18x12 with a 335/30/18, that would take up an extra 1.2" in either direction compared to my combo, so a little over a 1/4" clearance to the spring and quarters. The problem is, 6 5/8" isn't enough backspace for an E body rear. You need an extra 3/4" backspace compared to a B body rear, so that would put you at needing 7.25" backspace to make that rear combo fit. If you had a 68-70 B body rear in the back, it would be almost perfect at 6 5/8" backspace on a 18x12.
