What 600hp engine do you plan on fitting in an AD truck? If your rear ratios are 6 and 8 to one, probably can start out with an auto in the low ratio. Might consider fitting a Brownie trans in there, however a 6000 series is quite large. Original rear end might not stand up to that kind of
HP or RPM.

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.