Issue is access
My last boat was a 1990 35.5. The issue is getting access to the area where the original tank was located. It is behind the bulkhead at the aft end of the aft cabin. This is a solid bulkhead with no direct access to the area behind, where the tank is. This area is at the bottom of the hull, under the cockpit lockers (or swim platform lockers, can't remember which). Basically, it went in before the deck went on, and has to be cut out, as the poster did.I know of one owner who replaced his. He cut a large access section of the bulkhead in the aft cabin. Be careful cutting so you don't damage anything behind the wood panel. A clean cut enables you to replace the cut out panel later. He just trimmed the cut with teak strips, and it looked fine. With the panel cut out, you can easily fit in one of Peggie's nice plastic tanks.