It's kind of an "it depends" type thing... For the first gens, I think all the panels are available with high quality, good fitting replacements that are exactly like the factory panels. So ideally you can get the panels you need, drill out the spot welds on the corresponding old panels, and spot weld in the new one without a ton of drama.
Doing an entire floor, you will definitely need to brace the body in multiple spots to keep it from moving around once the old floor is out. Even more so if the floor supports and rocker boxes are bad too.
I would start by looking at the replacement panels online (or if you already have them even better) and examining your truck to figure out where things overlap and spot weld together, and come up with a plan for removing/replacing. I don't have a first gen to offer more specific advice, unfortunately, but this is the type of thing you will need to noodle on a little bit versus just following a how-to guide.