Backspacing and tire questions

[mechanic350]

ok so i have been looking all over for new wheels. i want to run 20x10 on all 4 corners and probably run 305/35R20 on the rear and if i can the same on the front if not 285/35R20. only problem is that i have found 2 sets of wheels both with 6.25 backspacing. the first wheel is the Boss Motorsports 338 style wheel 20x10 or the KMC Nova wheels. also 20x10. I have read most of the threads on here about backspacing and offset and wheel fitment. So far i haven't found much from anyone saying whether or not they have done this or if it will work. I have another set of bed fenders to do a mini tub job if i have to. and i do plan on taking my rear wheels that i have off to measure and see how it will fit preliminary before i buy the wheels. any help that can be given would be great. Currently i am running 18x8 in the front and 18x9 in the rear. thanks again for any help