I've seen this done and while not that difficult a swap, the real easy bolt-in for this is a 261 Chevy. The GMC will bolt right up to the 216 or 235 bellhousing this is the easy part.
Another real problem is finding a good GMC without a cracked head or block.
First what every car or truck the longer GMC is going into will need more room up front so see if you can re-mount that radiator forward. Also you can eliminate the GMCs fan and put an electric pusher in front of the radiator, saving a couple inches. Anything can be done if you want it bad enough to solve the few problems.
