Spicer 540 series OD trans. are somewhat rare, however to make OD, they take 3rd. gear so there is a big gap in ratios where 3rd used to be, so with a load, frequently stuck in 2nd because 4th(direct) is too large a jump, better if 2 speed rear, very awkward. Both Warner(IHC and Clark trans. have the same issue. The shift location for OD is right and toward panel.

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.