I was very happy with a tank made for my Bertram by these guys in Bayville, NJ.
http://www.speedytanks.com
The guys who did my SeaCraft retired.
My inboard is a 1976 and I probably have a diagram of the tank(s)if it helps you. Be sure to replace your vent line when you do the tank. It's a pain in the butt to do but worth it. I regret not doing mine.