New England Propeller in Plymouth, despite the misleading name makes fabulous tanks. (They primarily do prop work, but will also build tanks and repair skegs, lower units etc.)
I have had them do many custom one-offs. Some from a scabby drawing done by a boatyard monkey (me) on the back of a cardboard box!
Welds are beautiful.
Not the cheapest around, but you really get what you pay for.
Worth it for a top notch tank.