I agree, plenty speedy. The first numbers are the ENGINE torque capacity, the third number is the number is speeds, I. E., 3 or 4. There are no parts but bearings, so pull the cover to check. Having an integral brake: drum or Truestop is a plus, especially if you don't have cable brakes. A 6000 series is sort of overkill for a pickup, but duable. 5500 and 5800 series are great for pickups. Make sure to get the shifter mechanism, and driveshafts. $300. Is a good price as Grigg said. Check the innards first, treat them with care, use OD only with light loads, shift to direct with a load, not loading the cluster then.

God luck.

'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.