BlackPearl36C, most of the head plumbing in mine is above grade or behind a bulkhead at the worst. No floors to rip out. However, the later boats had a factory installed centerline holding tank at the aft engine room bulkhead. Again, shouldn't involve any floor ripping, but it may be a little awkward getting a waste line from the head to the engine room. Shouldn't be any worse than any other boat repair though.
admcmillan, I probably would like to do the same thing on mine. Same boat, same year. How big a tank were you able to get in there? Would you have a picture of the tank installation so I could see how it was all made to fit in that closet? It would be much appreciated.