Slush tanking- Glass boat

Greetings Gents.

I'm considering adding some slush tanks to my 33ft fiberglass troller. Right now, it has some totes sitting on the deck of the boat, and the hold has been converted to hold extra fuel tanks and be storage. It holds a massive amount of fuel, which is way overkill for what I need. (700gallons) I'd rather convert that fuel capacity into fish capacity.

This is what I'm thinking. I'm thinking of gutting it from the house back. Removing the 4 steel fuel tanks, and the 100gallon water tank. I'd have two 100 gallon aluminum fuel tanks welded up for the boat. Then, I'd glass up 5 slush tanks for the boat and put it all back together.

I realize it's a massive project, and I'm trying to minimize the amount of time out of the water as much as possible, and would rather chip way at the project over the course of the next year than haul it out and have to devote a month or two of work to it all at once. Do you think it's possible to glass up the tanks in my garage before starting the project, to save time when the boat is out of the water? 3 of the tanks will be 30w,30L,and 40D, and two will be 30w, 60l, and 40deep. (inches) Or, would it be possible to construct a good portion of the back deck of the boat, with the tanks, and then when I gut and clean the back of the boat, just lift the back deck with a forklift into the back deck, and glass it to the hull? That back deck of my boat isn't that big.

Also, what material would you guys recommend? I've got some experience fiberglassing. Do you think Nida Core is the way to go, or glass over plywood?

Any advice would be appreciated.