Replacing Plywood Hull Reinforcement Steps

[mstrader77]

I recently bought a 1973 19' Fiberform Malibu Jet. The 14 foot stringers sit on 1/4 inch plywood reinforcements that are 24" x 12' (one 4 foot piece and an 8 foot piece). It appears that the original plywood was coated with CSM on the bottom, set in place, stringers installed, then glass over. What is the best method to replace this? I am concerned with the working time of the resin on an 8 foot by 2 foot piece. Would using a balsa core be a better replacement? Should I glue the plywood down with PL Premium then go from there? The red line in the picture shows about where the plywood sits. At this point, I have separated the cap & hull and removed all the wet/rotten wood & foam and am ready to start rebuilding. Thanks for the help.