The GMC front cross member is a different design from the same model Chevy truck front cross member and radiator support in like years from 49-54. We GMC guys like bust stones by saying Chevy guys have long motor envy. That's not a dig, just a poke at some fun so don't take it seriously.
This might help you understand the key differences a little better:
http://www.oldchevytrucks.com/cgi/htmlos.exe/001932.4.2624903408900002722

and this one I took last year in Patrick's boneyard. He happened to have a GMC and Chevy frame stacked on each other (like in the photos above) in the yard:

You can see in the shot from the angle how the GMC radiator mount is a few inches farther forward and a little lower than the corresponding Chevy one.
This is a pretty common problem for what it's worth. Using the front GMC cross member will relocate the radiator and create other problems, so I'm not sure that will fix more problems than it creates.
Here's how Patrick Dykes manages the 302 front motor mount on a Chevy frame for what its worth:
