Could also be a 261. Dragging or pushing and letting the clutch out in HIGH gear works well, however before doing that, make sure there are no stuck valves. Oil the stems, tap rocker arm gently with rubber hammer or 2 x 4.

The brakes probably won't work, so make sure to allow for that.

Ed

Last edited by EdPruss; 05/21/2020 2:55 PM.
'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.