I believe those are 6 on 7.25" like I have, the only other truck that uses those are Divco-like in milk trucks. If you are handy, you could use two opposite studs, drill the brake drums and wheels to 10 on 7.25" and get some 19.5 or 22.5 wheels. I have thought of knocking the centers out of mine and seeing what tubeless wheel they would fit. Those wheels are configured to use as singles or duals on the rear, depending on fenders or not.

'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.