Jong,
I didn't see the year boat you have but, I can tell you this. My boat is 1980 model & I had to replace my tank also. My closeoff supports were plywood. The upright panel @ the rear of the tank had a hole for the drain fitting. There was a similar panel @ the front w/hole. There was a 1x1 board across the tank top with fiberglass mat adhered to the hull to hold it down. Along the tank bottom running lengthwise was a 1x1 glassed to the hull.
The tank itself can be ordered from BEMCO, they originally made mine for Catalina. There's a part no. on the tank. Very reasonably priced.
I carefully cut & removed the two ends using a dremel tool & removed the 1x1 top hold-down piece. I was able to reuse these & re-glass in. It's a lot easier than having to make a cardboard template & make new pieces. With supports removed you should be able to remove the tank thru the opening. This held true for me however, newer models may not allow this. Usually a lot of equipment like tanks may have been installed before the lower floor liner was installed in the hull.
If not sure, find a glas man. Take pics & measurements before starting & post them here for the guys to give you additional advice.
CR
