I am running a 62" wide (rim mounting surface to mounting surface) 88 Camaro differential with 15 X 8 rim with 4" backspacing and a 255/70/15 tire, on the rear of my 54-3100.
I would suggest that this is about the max. width tire you could squeeze in with this width diff and clear the bed. My 54 truck has a 50-1/4" wide bed. I did note some tire rub during severe corning before I installed my heavy sway bar. The clearance to the lip of the fender would be adequate to lower my setup but think a 235 tire on a 7" rim with the same backspacing might be a safer choice.
You might consider a panhard bar to tighten up the rear end, as there is bound to be some lateral movement during severe cornering.
I moved my diff. 1-1/4" back to center it in the wheel well. Made a 1"thick by 2" wide by 6" long block, drilled a hole on the bottom for the spring pack bolthead and back 1-1/4" on top, tapped a 3/8 hole for a bolt, that centers the diff-to-spring perch. The 1" tall block gives me a higher rake than the stock closed diff mount. You would want to add height to the block to lower your setup.
A word of caution....if you move the diff back, wait until you have everything bolted up solid before measuring your new driveshaft length. You CAN NOT just add the distance you moved your diff back to the shaft, and have it fit properly. Haven't figured out why...but you can avoid some frustration if you heed my warning! I've got a couple pics in my album.
I hope this helps you establish some sort of baseline for your modification.
