Our 1972 Catalina was manufactured prior to black water holding tank requirements. Clearly I will need to change out the seacocks and replace the gate valves with quarter turn ball valves. My question is this...has anyone used the newer flexible holding tanks? Is this a valid consideration or do I really need to put in a solid holding tank for the black water? I checked the forum archives but was unable to find an answer to my question. Thanks in advance for any thoughts, suggestions and/or sage advice and counsel you may wish to provide. From the looks of things under the dinette just aft of the head will be the place to install this tank because this is where the discharge line runs through the hull.