I'm in the process of replacing the last 4 feet of floor in my 73 overlander bath. You can see the progress here and in my blog and maybe answer some of your questions: http://www.airforums.com/forums/f46/...up-100164.html
Hard to say what needs to be replaced without some pictures. What does the wood look like all around the back perimeter? If the bolts are rusted, they should be cut and replaced with stainless. Post some pix for some better answers.
To remove the bathtub, I ended up cutting the plastic pipe with a long blade on a reciprocating saw. Somewhere along the line you have to cut it for it to come out.