GMC engines start the engine SN with the displacement, I e 228xxxx, stamped on a machined pad , r side, behind the distributor. It came with a 228, although up to a 302 could also be in there. Since the GMC logo is attached to the top of the valve cover, it could be a Chevy engine, couldn’t tell from pix.

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.