Wheel size and backspacing questions

[Jmiller022]

First off the boss lady shot down the 15s and whitewalls...lol

SOOO... Im looking at changing the 20s on the back to the gray boss 338s and the some 18" boss 338s in the front since Im having issues scrubbing the 20s on hard bumps.

My question is about the backspacing and offset.

Im looking at http://www.summitracing.com/parts/aew-3387-2850 on the back wrapped in a 275/40/20(29") that I already have and then Id like these http://www.summitracing.com/parts/aew-3387-9850 on the front with some 255/55/18(29")

Can anyone tell me if these will fit or if ill need spacers? My current 20s have a 7/16 spacer on the rear and none on the front.

thank you