The GMC engine is longer. That is the reason for the different crossmember. The front mount of the engine bolts to the crossmember. The GMC radiator sits further forward and mounts to the crossmember. If you put in a Chevy crossmember you will need to use a Chevy engine too to make it work. The GMC engine will no longer fit.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.