Is the 27 Continental an inboard boat, or an I/O boat? 10' beam or 9'6"?
Anyway, that is old enough that Tiara most likely won't have any records on. You are likely going to have to take detailed measurements of the tank and take detailed notes as to where the fittings are on the tank. Todd is a company that manufactures Polyethylene water and holding tanks. Look them up on the internet and see if you can match the tanks they offer to what you need. If they don't have exactly what you need, they can likely build you one.