I have a picture of the truck after the swap, but nothing while in progress.
We drove our truck to the pick and pull where they had a complete bed assembly ('81 I believe) still on the truck. I spent about an hour laying under the bed in the muck and unbolting it and disconnecting the lights and gas tank hoses, and then went out into the parking lot and did the same to my truck. I also removed the metal fuel fillers and the rubber hoses connected to the tanks.
The yard guy lifted the bed off with a forklift that had long forks, and drove it right out to the parking lot. He then lifted my bed off and put the new one in place. We jostled it around until the bolt holes lined up, and bolted it back on the truck. I pulled the light housings and harnesses off mine, since they were in better condition, and just plugged them back in. I think I drove it home before we reconnected the fuel fillers.
I no longer have the correct '75 tailgate design (in my year the GMC logo should be centered), and have fuel filler doors instead of just holes in the bed. The hinges for the tailgate are also different, but not obvious when its closed. However, IIRC they only charged me about $150 for the complete assembly.
Getting the bed bolts off can be tough, since the recessed square section under the head is supposed to hold it while you unbolt it underneath. For stubborn bolts, I was able to sometimes clamp a vice grip on top and use a section of pipe for leverage to hold it.
Bruce