Lots of possibilities there. First off, UPS doesn't sell their trucks, they scrap them. Who knows why, but....
One problem with a rollback, more than likely you would have to either shorten or lengthen the frame since rear axle position is critical on a rollback for it to operate properly.
What about building a Hodges style ramp truck?
This is a 54 that I looked about buying a while back. The PO built the ramp bed himself. If you have the room and welding skills it's actually not that hard to do. This one was on a IIRC a late 70's C-30 frame with 350:
