were can it go

we are building a 21 foot searay
the engine is inbord but the question is can the fuel tank be mounted in the back beside the engine

 

A common way to deal with fuel tank location is to put them as close to the center of gravity of the boat minus the fuel tanks as you can reasonably manage. This way whether the fuel tanks are empty or full they do not change the trim very much. If this is an in-outdrive as it sounds it would probably be best to locate the tanks further forward than at the very stern.