My ‘54 GMC has all the pertinent info on a tag inside the glove box.

I had a ‘53 GMC fire truck, put a 5831 Brownie in it where pump was, would go 70 MPH W/O load, drove it from CO to Iowa, etc. collecting gas engines, kept up with traffic, good mileage(12 to 15 comes to mind), only mod. to stock was .0625” off head with stock 302.

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.