Hi Ernie,
Never done a cockpit, but have done a lot of glass work over the years. Here are some ideas. First: can you visit a boat manufacturer to see what they do...I've been in a number of facilities in Maine,for example, and usually the folks are happy to show off their workmanship. Go aboard some local Hatteras to see what features you like.
Nidacore or other synthetic coring are an excellentway the go....but what's wrong with plywood? Just coat the under side (exposed to the bilge) with some epoxy and maybe a single layer of lightweight material to double the protective finish....it will last forever. I can't imagine why aluminum tanks could not support a cockpit floor....but glassing in some supports around the tanks to hold the flooring is no big deal.
The most important thing with adding any tanks is the specific grade of aluminum used, how they are fastened in place,the proper use of baffels to prevent surge, and proper cushioning against supports. It's especially important the supporting structure be flat and true before installing tanks and that proper cushioning be used. Likely some strapping arrangement is necessary to lock them in place...I understand Hatteras fiberglass tanks sit on the hull, foam under the tank, then mostly tabbled/glassed around the edges...maybe somebody can add more about that here.
I would suggest considering some way to drain rainwater and saltwater overboard from the cockpit. It especially collects in recessed hatch channels, if you use them, and then can run into the bilge...not a big deal but messy...
Consider your hatch covers before starting work; maybe the heavy duty commercial aluminum and waterproof style??? Aluminum might get too hot in the tropics and might sweat in your locale???(many types in the Hamilton Marine catelogue widely used in Maine for lobster boats.) Or you can make you own from plywood and fiberglass if you can figure out a waterproof attachment method.
My 1972 Hatteras 48 YF has a LOUSY arrangement: fiberglass channels under the floor (which are like a drain pipe and not saturated with resin against the waterflow) between the forward cockpit hatch and after hatch, then towards corner drains at the stern on each side. why LOUSY? because water sits in the channels and freezes and screws up the fiberglass floor!!!!...I have seen a number of old Hatt YF with badly deterioriated cockpit floors....mine is original and ok (I never figured out why), but the first thing I did after purchase was add a winter framework and cover so freezing rain and snow would NOT keep the channels wet and freezing all winter.
Good luck restoring your older girl!!!
