What is the reason for the new tank?
Does you old tank leak? I also had the idea the old tank in my 1986-31'needed replacing. It turned out the be the hoses, again and again. Usually the tanks in this model stay intack. Check out the hoses first, hose connections second, vent line third and internal head parts fourth. When you get tired of feeding new $10 per foot Sea Land hoses through a very tight place on the 31', do what I did. Spend $60 on a porti-potty. One trip to the mens room and no more head, hose, tank,or bio problems. You will spend more time sailing.