If you are going with the Nitrous I's, then they are custom built, and you can pick backspace.
In that case, on an 18x8 and 20x10 set, I would suggest a 8" with a 5" backspace and up to a 255 wide tire up front.
Note: For a stock suspension, I would run a 9 with a 275, and more backspace, but for an AirRide set up, and for the sake of being safe, I would suggest the 8" with a 5".
For the rear, a 10" wheel with a 5.75, and my suggestion... go with a 275/40/20. This is wide enough to look good. Will give you all the clearance in the world for when you drop that sucker on AirRide.
Note: For a stock, or slightly lowered car, I've done as wide as a 305 wide tire on a 10", but again, to be safe, and to set you up with a wheel and tire combo that will definitely work before and after your suspension mods, I would suggest the 275/40/20.
Fyi: the Nitto NT05 comes in both sizes you need.
Hope some of this helps....
