Yes the wheel backspacing comes into play a lot
When using an 8 inch wide wheel with 5 inches of backspacing you can pretty much stuff the tire wheel combo up somewhat in the rear wheelwell
BUT since the wheels you chose are more like 4 inches backspacing, they will stick out more
Where most times 2 inches ( 2 hockey pucks per side ) will work great you will have to go to 3 or maybe more, then likely you may still have some issues when off camber
which cuts down rear flex a good bit
Good way to make sure is with tire and wheel combo mounted on, shock removed, jack up one rear wheel, when it touches the flare bring it back down some, then measure distance between the stock bumpstop and the lower spring perch. Add that much spacer . Of course with the RC spacer kit, what did they give you for the rear lift ?
Just keep after your lift, keep measuring over every two-three months just to make sure it does not come down and start rubbing on your tires, front will be the worst and first to go