The flare doesn't matter. You'd think it would, but it really doesn't. Having run both the Duster and Dart fenders on my Duster (now a Demon tribute) I can say that the tire clearance difference between the two is pretty minimal. The Duster fenders have more restriction at the rear corner of the wheel opening, but the lower front corner and the fender lip clearance is very similar to the Dart fenders. I'm running the exact same 275/35/18's with the exact same rims and brake set up as I did when I had Duster fenders, they were as maxed out for clearance as the Dart fenders.
On that note, if you work from 73+ track width specs my 18x9's have an effective offset of 27mm (6.1" backspace). They just barely touch the frame at full lock. So, unless the RMS Alterkation with wilwoods is a 1/2" wider on the track width compared to a 73+ set up, you won't be able to run a 6.7" backspace.