Check out the following thread for a lot of discussion about floor replacments. My post number 10 has a list of some more threads where I describe the techniques that I used in more detail.
By the way it is a lot harder to get the body off of the frame with a 70's plus trailer because of the way that the channel around the bottom of the body is shaped. It is a u-channel pointing up and a c-channel pointing inward with the edges of the floor plywood inside of the c-channel part. Earlier units have just a u-channel that sits on top of the plywood making it a lot easier to lift the body off.