I had this same problem on my 1988 Catalina 34. Your old tank is too small to install the hew type of valve. You will need a new tank. If you happen to have a Seaward locker and if your problem is getting the new tank to fit in the old locker see the message below that I got from a fellow Catalina owner:
Seaward, who makes the propane system which Catalina uses, has a fix for this. They have a new molded cover, which accommodates the higher tank with the OPD valve. They offer this at their cost to upgrade earlier systems. They also have some shorter 1/2 inch long stem adapters which go between the POL fitting and the "T" to the gage and regulator so you can use the newer tank valves with the inside and outside threaded POL fittings. Believe the short stem adapters are only about $5 each but Seawards minimum order is $20.
New lid and adapter kit for the higher OPD tank is Seaward PN 80233. The OPD tank is Seaward PN 93270. Contact Tom Shultz who is the owner. He gets results and others there often don''t. Phone is 562 699-7997 and Tom''s extension is 204. Another contact there who knows the equipment is Lonnie at extension 213 but it''s usually a lot more difficult to find him available.
This is what I did and it worked perfectly.