Fish jack-
The bottom of my fuel tank looked like yours were the rubber was. As for marine ply for the fuel tank support you really dont need if you use high quality plywood it will work just as good. However I would use 1808 biaxial to cover the top side and bottom side epoxied to the plywood. Router the edges facing the bow and stern so you can wrap the fiberglass over the exposed edges and that will last another 30 years. Anoter option is to go with a composite such as cosa and that would be first class. while you have your boat gutted and exposed you may want to check out those battery boxes (flat surfaces on either side just behind the gas tank) they were rotton in my boat. That is one of the only places you will find wood in your seacraft besides your transom and hatch coring. Keep up the great work.
Battery Boxes:
http://www.casdvm.com/photos/SeaCraf...ttery%20boxes/
strick