I have two plywood seams, one 4' from the engine bilge wall, the other four feet forward of that between the front seats. I used 3/4" tongue and groove so no support required underneath. I used Marine flotation foam under the sides, and made and glued new rubber supports under my tank. Good time to work on the belly tank and insure it will last. A drain channel now exists bow to stern in the center. I hand fitted all the wood peices before doing any glass work. Everyone has their own way. One thing I remember well, fiberglas material is not as flexible as one might expect, I chased air bubbles for a long time it seemed. My advice, do glass work in small batches, maybe a pint at most, the results are worth it.