I owned and drove a 66 C30 panel for 25 years, and if I remember correctly the floor was planks, not plywood. That's a long truck you have there and the body flexes a LOT. Although it isn't the easy option, you many want to consider using planks instead of plywood as the planks are not as prone to flex or bend. Thers should be steel runners between and on top of the planks to strengthen the floor (and the truck as well once it is all bolted together) and also bolt to the body crossmembers. I am guessing that with a plywood floor replacement done long ago before you got the panel, those strips were probably not reinstalled.
