Highest ratio available might be 6.6 if you can find one. Might be best to find a modern rear axle(2speed) with juice brakes, although that bolt pattern is uncommon on modern trucks. There is a six lug bolt pattern that is common, so, if you find a whole truck, would be able to use two matched axles with tubeless tires. Just depends on how original you want to be and your intended use.

Best use for the pump assembly is to replace it with a Brownie. Takes care of too low rear axle plus low gears for creeping,

Ed

Last edited by EdPruss; 05/04/2016 2:25 AM.
'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.