Old farm trucks, like yours, running or not, many wrecking yard people won't know what you are talking about, just have to dream up some other reason to get into their yard and look around for yourself. GMC engines have the displacement as the first 3 numbers of the ser. no., starboard side, STAMPED on a machined pad, behind the distr.

'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.