If the panel plywood floor goes in like the suburban plywood goes in, I think that two different techniques could be followed (I have done neither - I have read about them):
Technique 1.
If you want to use one piece of plywood, it is wider than 48". A marine/boat repair shop might be able to come up with marine grade plywood that is wider than 48" - maybe 54" x 96"
If you want to use one piece of plywood, you will have to remove one of the "side rails", by cutting out 20-28 spot wields (someone counted them long ago).
Then the fitted plywood is put in place and the "side rail" would be welded or screwed to the sidewall (into the original position).
Technique 2:
I think that Weeds might have joined two pieces of plywood with some sort of lengthwise joint. I think that his method did not necessitate removing a "side rail"
I think that Weeds posted his technique many years ago?
