A gmc is a few incheslonger than a chev but that can easily be over come and has been done many times. You will need a civilian oil pan, Carb, ignition and generator. The military ran 24 volt. Also most of them ran automatics and had a different bell housing. There is a boss on the block that has to be ground off to accept a civilian starter and the military blocks do not have a place for a fuel pump so you must use an electric fuel pump. The best is to have a donor engine for all the parts you need. I have leaned toward GMC for 50 years, but there is nothing wrong with a 261. It's a lot easier conversion.