To attach a GM 4500 to a 465 or 420 bellhousing, just make the retainer fit by boring out the bellhousing, making a spacer, turning down the retainer, or some combination. Mount the trans. up to the bellhousing off the engine, mark the existing bellhousing holes in the 4500 trans., drill and tap the new mounting holes in the front of the trans., make the bottom ones blind, or seal them up so you don't get a bath every time they are removed. If using a bellhousing w/o a bottom access hole, must mount the trans to the bellhousing before the engine.

'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.