We used 3/4" marine grade plywood on our '56. The reason we went with 3/4" instead of the 5/8" (actually 11/16" to match the original) was we took out is the old floor AND flooring which went under
the c-channels and since we were doing a shell on installation and couldn't get the flooring under the channel with the wood. So to make-up the difference, we upped the plywd thickness.
I second the opinion to not
use OSB or MDF...two reasons, 1) the water issue previously mentioned & 2) the additional weight. They are much heavier than plywood because of all the glue.
We did not need anything wider than 48"...the seams did not overlap in ours, the elevator bolts spanned the gap between the panels. When we did ours, we attached to the frame on both sides of the joint. the butt joints will be filled prior to installing our flooring.
To see some pics, check out: http://www.airforums.com/forums/f91/...tml#post558457