Well you’re talking to a guy who has done two of them, so I don’t think, I know.
Go look at your truck and tell me if you can cut it so you have 2 1/2 more inches of tailgate opening width on one side. It’s literally next to impossible, and I’m a car building guy who has turned hardtop ‘50s Cadillacs into convertibles that look factory no matter where you look.
Cutting the truck instead isn’t a realistic option unless the truck is junk and you don’t care how much you hack it. It’s not just sone sheet metal inside the box that would have to go.
If I were going to do it, I’d pie-cut the bed floor and un-taper it, adding in a 5” pie slice from a donor bed floor, then redrill or otherwise modify the bed mounting bolt areas. Then I’d cut two tailgates and weld together one 5” wider version, or just make a custom 5th-wheel style gate.
The interference area is usually the bathroom. Sometimes there’s enough room to “clearance” it without disturbing anything inside.
On my current one, you’re right the shower wall was right up against the kickout area. Some careful slicing and fiberglassing gave me the two inches I needed.
The other thing you CAN do, although it will look funky, is just swap an earlier model square-body bed onto the truck. In the late 50s and early 60s some pickups came from the factory with mismatched beds, because they wanted to get on the “fleetside” bandwagon but didn’t have their own tooling yet.