My rib has a 100 litre stainless tank built inside the console. The tank must have been fitted and then the console fibreglassed on around it. You'd never do anything to the tank without removing the console 1st in my opinion. Keeps the boat nice and tidy but you'd want a lot of trust in the tank not to crack / leak etc, don't buy a cheapo one! Some folks fit the fuel tanks under the deck, but in my experience the rib is quite light on the nose even witha puny 90hp 2 stroke. If I went underdeck I'd want a fuel tank up in the bow area, don't know how the hulls are made up on these ribs though, why don't you try calling Barnett Marine, I think they deal with these ribs a lot?