Serial number on GMC engines starts with displacement, i. e. 302xxxxx. There are 4 different GMC heads, identified by numbers CAST in top, seen without removing valve cover, come to fit certain pistons; therefore all heads will not work with all pistons. 2 have small intake ports. Some rare stock intake manifolds come with 2 barrel carbs(military?)

Patrick's catalogue has Casting numbers, if you have one of his old catalogs, can't find mine now.

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.