Well 3B basically covered it...
That front crossmember is the only real difference between a Chevy and GMC frame. The crossmember is different in several ways that might matter to you.
1) The motor mount is further forward to support the longer GMC motor.
2) Because the motor is longer, the radiator is mounted further forward, and its supported by this crossmember. Since all the front sheet metal mounts off the radiator mount it affects all that stuff.
3) There are original factory sway bars available and since they mount on this crossmember, you must use a Chevy on a Chevy or a GMC on a GMC, though I suppose some creative work might adapt one...
It would take removing a bunch of rivets to swap crossmembers, but I'm sure it can be done.
As for the motors, the GMC is all of the things mentioned above. But the larger 270 and 302 are the only ones considered desireable, the smaller 228 and 248 seem only of interest to serious restorers.
