D-80's can have up to 3.54 gears,carrier swap, reqd.
The solid front axle from the HD 3500 can fit, check spring pad dimensions- spacers can be made to correct for smalll discrepancies- see Grigg. Probably can use the Saginaw steer box and pump from the 3500, might have to remove skip spline from arm, change steering from side to side to fore and aft-just remount the box. While you're there , get the Hydroboost.

'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.