Appropriate Wheel Offset for 18" Wheels?

What about something like a 265/40/18? Just a touch taller (probably no bueno for being lowered) but skinnier? Or a 265/35/18 (might look a touch funky because it will be slightly shorter than the front)? If needed be I guess I could go with some 255/40/18's in the rear and keep the 275/35/18's as a mach up tire for the future. Tirerack has some Hancook V12's on closeout for a touch over $250 and they're rated for an 8.5-10" wheel. I guess 1/2" shackles need to be on my list for things to buy in the future.

I don't think a 265 will fit with the stock spring location either. When I mounted the 275's on my Dart for mock up, they were physically in contact with both the quarter lip and the springs at the same time, to the point that the wheel was obviously dragging on both sides when rotated. 5mm of space per side isn't going to cut it for clearance, and I don't think you'll be able to get enough out of cutting the quarter lip to make them work on the road.

A 275 really needs that 1/2" offset, and even then, its still pretty much maxed out with the quarter lip trimmed back. With the stock spring location, I think a 255 is as much as you can safely run. And depending on the tire, car, and spacing, even the 255 might take some trimming.