They don't make a 48RH, they make a 47RH, find a 47RE, that would be easiest. A 48RE would work with some modifications. You need to get a bushing for the neutral switch, and then change the VB lever on the VB.
allison at 545 is pretty beast. I put one in an old 94 12 valve dodge dually I had. Nabbed the drivetrain out of a school bus. Gotta nab that sae flywheel adapter and the flexplate, starter, tq converter, etc then you gotta change yokes on your driveshaft to mate up to the back of the tranny. it was slow though, had 5th gear that was direct drive haha that with 4:10s in the rear....