Bill, thanks for posting the '72 catalogue. Behind the 503 in my '54 GMC, I tried to figure out the correct PTO, never could find the right one, while wasting time on Ebay one night, saw an idler gear for PTO's, wondered if it would work, bought one, tried it out, worked, what luck, now my crane works.

Ed
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'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.