Not a expert, but USA GMC's came from the Factory with different engines than chev's. The 6 Cylinder engine was a 270 and the V8 was a pontiac. Yes Hydromatic tranmissions were available in GMC trucks.

My GMC is Canadian and has the same running gear as a Chev would have, ie 236 I6 or a 283 V8


Blaine Dumkee
59 GMC 9314
Fort Smith NT
www.flickr.com/photos/northerngmc/