For what it's worth I used 3/4 pressure treated plywood on my 53 truck, and it has been there for 5 years.
I must point out that my angle strips along the lower edge of the bedsides were rusted to peices, so I replaced them with 1/8x2x2 angle iron, which makes them wide enough to cover a single sheet of plywood. After I let the pressure treating moisture dry thououghly (a couple of months) Iused Thonpsons water seal. Since I actually used the truck for work, I eventually got oil stains on the plywood and it looked bad..
This summer, I figured what have i got to lose, and applied a couple of coats of Mahogany stain. Didn't get the reddish color I hoped for, but the floor is pretty much all the same color, and the oil in the stain gives additional protection
