X2 on what zieg said. If its out a cab & chassis its a 14 bolt. If its out of a dually its will be a Dana 70. You said dump truck, I am assuming it is a 14 bolt out of a cab & chassis. If so the rear end housing is narrower than the pick up housing. If you plan to run single wheels they are going to be way inside the wheel well, possibly even rubbing on the inner fender well.
Any particular reason your swapping an 8 lug rear end into a 5 lug truck?
A 12 bolt should hold up the Cummins if you respect its power.