Early on GMC used Olds and Pontiac sixes if I remember right. I think from the flathead era, though I'm sure not an expert on those.
The light truck GMC overhead valve inline sixes are as follows:
228
248
270
302
I believe there are military versions of both the 270 and the 302 and both were available pretty early. The 228 and 248 are the standard motors in '47 to '54 1/2-ton stuff. The 270 was used in pickups starting in the late 50's I think.
When GMC started installing V-8's in pickups they used Olds and Pontiac motors again.
While the 2-ton was the largest truck Chevrolet made, GMC made bigger ones, and there are larger GMC sixes out there. I know little about them.
About '63 GMC introduced V-6 and V-12 motors which replaced the inlines in both the light and medium duty trucks.
A complete and accurate application guide would be a great thing to read. I sure don't have all the information, but the first place I'd look would be OldGMCTrucks.com.
