If you're keeping the small bolt pattern then you should probably use 4.25" backspace in the back, not 4.5". With BBP axles the 4.5" is the way to go, but with SBP it will probably put your tires pretty close to the springs. With 255's you'll have room to the quarter, so the 4.25" backspace would probably work better for you.
The front should be fine with SBP disks.
