Beds are pretty much identical from 75-80. The only difference is the number of hooks on the bed rail and some changes to the gas filler neck. 75-76 have 4 hooks per side with 3 on the tailgate, later has 3 per side with 2 on the gate. The filler neck is not a major change, but some caps will only fit earlier years or vice versa. I think that changed in 78, but I'm not certain.
I'd think your best bet is to look at salvage yards. There is a place in Taiwan that advertises they make a replacement bed, but I've never heard of anyone actually buying anything from them. If you have a source for new stepside beds, please post it. There are a lot of folks around here that would be interested in that. You might find a great deal on a fleetside bed if someone buys themselves a new stepside.
Make sure you disconnect the all the hoses leading to the gas tank from the bed before you take it off, and don't forget the wiring harness connector.

If you are going to replace everything just like it is, mark those hoses leading to the expansion chamber in the bed. There is not a very good reference that says which goes where.