Your best bet for reproducing the shape of the floor sections is to use the body itself. Can you lift up the body enough to get a piece of cardboard or masonite under it, to trace the edge? If not, you can trace the edge of the C-channel, and then add the thickness to get the final dimension.
As for material, it is probably 5/8" plywood, but your best bet is to measure the thickness of an actual piece of floor that came out of it. It may be 3/4". It is important that you match the original, or the holes in the front hold-down plat won't line up again (assuming the plate hasn't been replaced in your reconstruction).
There is a general preference for using marine grade plywood, but I am not convinced this is a necessity (I used the marine grade, and it was around $75/sheet for 3/4"). Whatever plywood you use, you should seal it up, at least along the edges, to prevent a repeat rot-out. I used a few coates of polyurathane.