Ideally you want to put it under the deck to keep the centre of gravity low, and have it as large as will fit in the space.
This is a substantial project, so you may as well maximise your range -- you don't have to run with it full all the time if you don't want to, but at least you've got the option. Anything to avoid refuelling at sea from jerry cans . . .
John
__________________
|