Welcome to the Stovebolt Jesse!
GMC's with the standard cab had three different fender/grill configurations. I don't know the breakdowns, but basically 1-ton and smaller trucks are the same as a pickup, the next level is the same as a Chevy 1-1/2 or 2-ton, and the 3-ton (450 series I think) has something even bigger. Then there are even bigger trucks that have a slightly different cab in addition to different fenders, hood and grill.
The significance of the fender size is that the track width on a 60's truck is enough wider than the '52 or '53 that unless you have the biggest fenders, keeping the wheels under the fenders will complicate the swap.
What are you going to do with the truck? Unless you intend to haul heavy loads I'd stay away from the added weight of the C-60. Not only does it weigh more, but brake and suspension parts are likely harder to find. Also your intended use might suggest other ideas.
The '63 to '65 frame will have a coil spring IFS that can easily be swapped with a '71 to '87 that would give you disk brakes. That might be another consideration. Mounting the 261 in the later frame may require fabricating some mounts, but if you are contemplating a frame swap you must already be comfortable fabricating mounts.
Another thought would be to look for an earlier frame. They are not that hard to come by in my area.
