In your model age group the u-channel at the bottom of the wall sits on top of the plywood which extends pretty much all the way out to the edge of the body. I believe that what you suggest about replacing the floor without removing everything is possible but there are some tricky things about it. You will have to build some sort of framework to make sure the body stays above the frame rather than settling down when you take out some of the plywood. Also, as already mentioned, there are a lot of fasteners holding things down both around the edges and to the middle framework. It is harder to get to parts of the frame for repair and painting.
What I did on my '73 to remove the plywood was to take my circular saw with the blade set to almost as thick as the plywood. I cut along the cross members (look for the screw lines) and as close as I could to the edge of the body. Once you remove the big pieces you are left with narrow strips around the eges and along the frame cross members. I then used a variety of tools to remover or cut the fasteners including hacksaw, sawsall, air powered cutoff wheel, etc. I was able to come up with a simple support system that you should be able to find details of in the threads referenced below.
I did as you suggested about inserting the end sheet of plywood. For the others I installed my sheets lengthwise instead of cross wise. I put a spline of plywood down the middle in between the cross members and used glue/screws along the joint. The techinique works fine.
Check out the following thread for a lot of discussion about floor replacments. There is a lot of detail here in the forums for both body off and body on techniques. My post number 10 has a list of some more threads where I describe the techniques that I used in more detail.
Removing the body shell and installing new floor completely - any one done this?
The fourth listed link and my post #74 has a lot of details as to the type of tools needed and some tips and techniques you might find useful.
Feel free to ask for more details if what I suggested is not clear.