I couldn't wait any longer and went ahead and installed a sheet of 3/4" marine grade plywood. ONE PIECE!! It was relatively painless to do but the way i did it it took me the better part of a day to do. Like I mentioned I only wanted to do this once so I went ahead and spent 15$ on a sacrificial piece of underlayment that was going to become my template...
But before I began replacing the floor I decided to clean up the exposed frame with brush wheels and sanding discs. Then I primed and painted everything I could see. Rust oleum primer and flat black. And while my belly pan was semi down I could see the best location for the two rear stabilizers that I also installed during the frame painting project.
Anyways back to the template. With all the old bolts and screws removed from the underside of the exterior walls I cut the underlayment to length and was able to slip it under the shell and butt it up to the existing plywood. From the outside I traced out the circumference of the shell and cut the piece down until she fit perfectly. After that I transferred all the old bolt holes with a sharpie from above and below the floor through the old holes in the steel and onto the template.
I clamped the template to the marine plywood making sure it was square and began jigsawing. Using the template as a guide I just ran the saw down the piece making sure I kept blade contact with the template but didn't actually cut into it. Next I drilled out my sharpie marks for the 1/4" stainless carriage bolts. Then I flipped the 3/4 upside down and reclamped the template to drill the underneath marked holes.
I wanted the piece to slide right in when the time came so I chamfered the top edge all the way around the outside edge. Also I routed out 1/16 of wood from the topside of the newly cut floor 2 inches in to accommodate an ice and water shield treatment I thought would prevent future 1970's rear bath floor rot without increasing the thickness of the floor. Next, I started at one end of the sheet and worked my way around applying the "grace" to the top of the plywood right up to the routed edge. When I got to the curved sections I had to slice into grace to create an overlapping seem sequence in order to keep the grace in one piece all the way around the plywood.
Street side was looking to be the most unobstructed side to slide the piece in from due to the missing hot water heater. With a little help from my awesome girlfriend we managed to get it started really nice. As the resistance gained pushing and tapping with a rubber mallet turned into full blown mule kicking and whacking. Without any damage to the grace she was in place in under 20 mins. My heels and arches of my feet where killing though after.
Miraculously enough the bolt marking trick worked with precision and all of them are installed plus a few extra along the outside. I know the bumper is the cause for the wood wicking up a lot of water and eventually rotting out the floor so I think I'm going to reinvent the rear a bit when the time comes.