Its 80ish miles one way to the oil rigs. Not knowing the fuel consumption of the 2.5L 200 Merc, I want to overkill on the fuel capacity. I want a tank mounted in the belly where the livewell is now. But i would like to install it while the cap is off. 
But I like Rednecks idea of using the in-floor compartment in the cuddy. 
Tsubaki, I would have to raise the floor around the 82 gallon tank to make it fit. To me its too much work, plus I am building two fishbox, tackle storage, leaning post, and rocket launcher units, (one on each side). I will wait for a better fitting tank or while i have the cap off have one made. I really need to know the condition of my current 60 gallon tank first. 
I already have a 16hx12wx36L 27 gallon tank that i need to find a place for. 
If my current tank is in bad shape, I will probably have a 100 gallon belly tank built. If i get far enough in welding to feel confident in aluminum, I might just build my own.