We did it.
We just re-did our entire waste system in our '90 H35.5. Replaced the tank using a stock 35 gallon Todd polyethelyne tank from West Marine (approx. $125). Access to the tank was made by carefully cutting an opening through the aft cabin bulkhead. In order to fit this large a tank, we built a platform in order "flatten" the hull curvature. While in there, we also replaced all of the hoses (did'nt want to have to return to the scene for a long time).Good luck on yours.