The bed I have on my '75 is from about an '81 truck and all the bolt holes lined up when we swapped. We unbolted the bed, and disconnected the wiring and filler tubes, and the yard guy used a fork lift to lift up the entire bed and set it on the ground. He then did the reverse with the "new" bed. It only took about 20 minutes to make the swap.
I also had to get the gas tank filler hoses and metal filler tubes from the other truck, since it has doors to cover the fillers and my original bed had the fillers directly into the bed metal. The tailgates are also slightly different, so make sure you get a complete bed. In my '75 the GMC is centered on the tailgate, while newer trucks have it to one side. If your truck has a bright trim panel in the tailgate it will be designed differently on the new bed and holes will be in different place, so its probably best to just use the new one.
When I bought locking gas caps I had to remember that I had the newer style fillers (caps thread into the filler tube), since the old ones on the truck were the half-turn gas caps. Most of the side trim can be transferred over, but you will need to fill some holes and drill some new ones if you want it to match the cab. I bought a new bed trim set from Truck and used that to make it match.
Bruce
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