GMCs didn't get GMC engines in Canada in those years
that truck originally had a Chevy full pressure 235 in it, one year before USA Chevys got the full pressure 235 in regular production
my understanding is that the GMC engines that the USA trucks have are slightly different length and so in the USA you can't swap a Chevy engine into your GMC because the front frame parts are slightly different to accommodate the respective engines
whereas in Canada, the only different between a Chevy Truck and GMC for AD years is the front grill and radiator, the tiny name plate on the dash, and a GMC horn button (there maybe a few other trivial details i'm forgetting at the moment, but they are not significant in anyway to the operation of the vehicle).
you can (and i have) swapped the Chevy engine from a GMC into a Chevy, because it's the same engine in Canada
i have never seen the GMC engine up here in person, doesn't mean it doesn't exist, but it's rare if it does
-stan
