15x10 fitment on non-tubbed Duster

If you want to use that size keystones, with that little backspace, you are definately gonna have to narrow the housing to suit.
Personally, i would keep the relocation kit, get a cheap summit or jegs exhaust system so you can use your exhaust tips, and then get,or make, a custom fuel tank/fuel cell to work. Imo the best way to get a ten inch wheel under it.
(Put the tire you want to run on the wheels before you do any measurements).