If it's going to be a work truck and carry loads and possible trailers, a better option than the 420 would be a 5 speed trans., i.e. NP-540, Clark, etc., don't necessarily need OD for a V-8, just determine avg. loads, see what gear ratio you have now, tire size-you could go w/ modern 22.5's rather than 20's, 19.5's look too small. Perhaps 4.11 or 4.56 ratios, depending on use, use what you have first, then select different if necessary.

Above and other trans'(465) come w/ GM bolt pattern, however, other patterns exist that can be used. Longer input shafts that use larger clutches may be used by obtaining approx. .25" thick GM adapter and associated bellhousing, if desireable trans. is obtained w/ other bolt pattern. PM for px.

Rear mounts using squared U shape stock GM crossmember w/ 2 rubber mounts, single front mount, or front stock GM front crossmember using 2 rubber mounts on the frame, 1 rear mount under trans. Not a good idea to use 2 front and 2 rear mounts-frame needs to flex.

'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.