I think the best bet for your '40, would be to change to an open driveline, lot of work but there are lots of unused rears and trans' around, might as well get an updated rear, two speed might get slightly better ratio(6.5:1?) so can fit in a Brownie while doing the work.

Is your wheel bolt pattern 5/10 on 7.25" or ? Could use 420, 465 4 speeds or 540 5 speed.

Good luck.

Ed

'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.