Justhecks,
I reocmmend that you check out this website address:
http://www.yachtsurvey.com/fueltank.htm
Regarding the plywood "deck," 5/8-3/4" ply should do it. Just be sure that it is well sealed with epoxy; up to and including a vacuum bagging of the the epoxy application. After that about the only thing that may cause a problem is sand or grit getting between the treated/epoxied plywood and the inside of the hull where it will come in contact and the areas of contact between the plywood and the tank.
The weight of the tank with gas in it should not move much and neither should the plywood and the inside of the hull. Only if the wear at these contact areas become severe enough to penetrate through the epoxy sealed wood is when bad things begin to happen.
Good luck.