I spoke with CST a few months ago because I was considering running the spindle with my 18" Addicts. They will tell you max backspacing is 5" and it won't work. There was a member running Moto Metal 951's with the CST spindles though, and they have even more backspacing than we do with the Addicts. He posted pics of the set up and you can see there is MINIMAL clearance. You may be better with the 20" wheel than the 18's, but I think you'd run into issues with the 12.5" wide tire too. If you know you want to run the spindles I'd order some 1/4" or 1/2" wheel spacers and go for it. You may find you have adequate clearance and don't need the small spacer. If you need the spacer you can run a slightly wider spacer in the back to maintain running width front to rear.
2008 Red Brawn Crew Cab SE, 2wd
Mods: Pathfinder switch covers, chrome westin nerf bars, OEM fog lights, 18" XD Addicts w/ 285/60 Cooper zeon ltz's, Bilstein 5100's at 2.0", 1.5" PRG block with 5125 Bilsteins