GMC engines are about 3" longer than Chevy's, so, you will need to allow for that, GMC's move the radiator FWD in the core support.

PTO's are transmission specific, so a 420 PTO may or not fit a 465. Since you have both. Straight cut vs. 'slanted' cut gears and also depth in the gears(shims are available to depth of engagement) are some issues. I am sure there are PTO shops in IA.

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.