It's been 11 years since I restored my Compact II, but I'm pretty sure I used 3/4" or slighly less exterior plywood, 4 x 8 sheets, for my floor. I wrote in my thread what kind of adhesive I used for joints and edges, but I also think I used a floor levelling compound at seams to get smooth transitions. Then primed and painted with a good floor enamel, then put down sheet vinyl flooring only with adhesive tape. To this date I have no floor problems; the vinyl lays flat as a board. Durable, too.
I never did an off-frame job, but did weld a couple of cross members for more floor support.
Hope this helps.
'74 Compact II