I also used the original '37 GMC frame, added GM P3500 HD rear axle w/ my own air ride(two bags off Kenworth 4-bag axle), front axle and Saginaw power steering. Very easy to do, although GM mounts the front springs with a tapered planform, had to parallel modern springs-used longer Ford 2 leaf, moved shackle mount to rear of springs, front spring mount rigid(reverse of '37). Longer springs give softer ride. Wider front axle taken care of by moving front fenders out 1.5" each side with Al spacer blocks.

'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.