Engines are different, chevy engines are shorter and the rear mounts are different. I actually have a 54 GMC 100 w/ a Chevy 261 and SM420, which originally came from the factory with a 248 GMC engine and a hydromatic tranny. The gauge cluster is also different between the Chevy and GMC. A chevy anti-sway bar does not bolt right in to GMC without an adaptor because the front cross member is different. The transmission cross member is different as well. Slash aprons are different. There may be other things different, but those are the ones that come to mind.
Jim
