I hope the front crossmember isn't different, but I don't have a GMC to compare to. I know the radiator support is different. Time to start parts hunting!
The cross member IS different, but I contend that it only affects the engine mounting (GMC engines are longer) I wrecked a 1953 GMC Suburban in the 1970's and I had already put in a 235 engine. I had welded an extension bracket to install the front motor mount on the 235. I replaced the front GMC cross member with a Chevrolet cross member so I could have the correct front mount for the 235. I bolted the new GMC front sheet metal to the cross member and I do not remember that I had problems installing the radiator. Remember this was a time that no one told me I could not do it!
There was a guy on another forum that said that he wanted to make a GMC hood out of a Chevrolet hood and all the experts told him he could not do it, he did and now they are all fans of his build
I did look at some pictures that Rob English took of the two cross members, there is a definite difference, but like I said it worked for me.
