My rear wheels are C-5's that are 9.5" wide(total rim width is 10", with a 275/40-18 tire mounted the total width is 11"). The distance from the inner wheelhousing to the inner edge of the quarter lip is 12 1/8". I might have go with a different size tire to clear or roll the quarter lips.
I really think this can be done w/o mini-tubbing, but I will still need to test fit the 18's with the tires mounted to be sure. I'm with you on mini-tubbing....if you have to go through that much work...I personally would have justify the work by putting some big rubber back there!
With a precisely measured spacer-the wheels will fit especially if the lips are rolled. **When I refer to "spacers" I am not intending on using a five-holed "washer", but a high strenth machined billet spacer with hardened studs. These are in essence designed just like adapters, but with the same pattern from hub to wheel.
The 17" C-5 front wheels are are only 8.5" wide, and will be no problem at all.