Cut off the flange on the top so that you can fit the plywood against the shell. In my opinion you should cover the replacement piece with fiberglass top, bottom and edges.
After it is fitted you can replace the fiberglass tabbing from the top.
Personally I used epoxy, but some insist on polyester as that is what the original shell was made with.
Currently you have the most support of the rear shell removed and it is probably resting on the frame where it passes under it. At least you do have a complete floor ahead of the bad part.
You could install a support the the underside of the existing floor (I would epoxy glue and screw it under the existing floor so that the junction would be supported.
Glue and screw the new rear part of the floor on the top of the support.
What you have is the result of not waterproofing the top of the newly replaced floor and the inevitable
leaks let water collect on the top and rot the wood.
These trailer floors rot from the inside out.
Three underlying reinforcements under the remaining floor to tie the old to the new would do. I would suggest making certain that these were waterproofed as well.
I used an oscillating saw to cut the tabbing from the top, using the bottom as a support for the new. I used strips of fabric overlaying the junction, epoxied in place.