With a GM 4500, stock GM input shaft-Dodges are about 1" longer, one CAN use the original bellhousing. If the retainer dia. isn't 5.125", bore it out; temporarily hold the trans. against the bellhousing, transfer the bellhousing 4 mounting bolt pattern to the front of the trans. Drill and tap the holes in the front of the trans., best done w/ the trans disassembled, but can be done otherwise; make the bottom holes blind or fill w/ epoxy or something, so you don't get a bath when the bolts are removed. The trans. must first be mounted to the bellhousing w/ accessing the mounting bolts through the bottom.
Good luck.

'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.