If I was going to use plywood as a floor I would use MDO plywood.
it's not cheap but it is used for exterior signs and such.Probably $60.00 for a 4x8 sheet of 3/4 inch.



These old bolts are in my blood. Hard thing is focusing on just one.

1937 Chevy 1/2 ton panel
1953 GMC 2 ton. future car hauler