Appropriate Wheel Offset for 18" Wheels?

Did you do a spring offset? Or are your springs still in the stock location? From the pictures, I would say you have less room than what you indicated. Tape measures are lousy for getting measurements that small, and you want to make sure everything is lined up straight. The tires having some bulge to them makes good measurements tricky. I know it seems nit-picky, but when the tolerances get that tight it matters. To get your spring-to-tire measurement, use a straight ruler and slide it to the tire across the top of the leaf spring. For the tire-to-quarter measurement, use a straight edge across both outer edges of the tire (I use a cut down yard stick). Then measure from the straight edge straight across to the lip. That one's even more important if the quarters are still above the widest part of the tire (at the bulge). Pictures can be pretty deceiving because of the angles involved, but I don't think you have enough room to keep from rubbing once you're driving down the road. For example, if you look at this picture you would think I probably have at least an inch or more from the quarter lip to the tire on the back of my Duster. But measured with a straight edge, keeping everything parallel, I really only have about a 1/2". Same for this on. The 225/60/15 on the cop rim in this picture actually rubbed on the quarter lip under compression (speed bumps). Not much, I never actually heard it, but it left a scuff on the tire.
The springs are in the stock location and I know there is going to be some modifications needed to make this work. The tires do have some bulge to them and I know the pictures are deceiving. If I trim the lip a bit and reduce the offset I might be able to get a little more room as I'm definitely not happy with the inner spring clearance. I've also got to check the wheel wells as I'll have to do some "massaging" to get it to work. It just looks so close and too good to be true.