Holding tank or Fuel for Jeanneau
I hate to say this at this late date but stainless steel is an extremely poor choice of material for a holding tank as the acids in the waste burn through SS rather quickly and allow odors to escape and eventually leaks to occur. Heavy wall plastic is the recommended material for holding tanks.
Also, when full a tank that large is somewhere around 900 to a 1000 lbs. That is an enormous amount of weight located right up in the forward end of the boat. Even for a 51 footer this is critical area of the boat to carry that much weight.