I believe my tank (1984 200ccp) was a ~73gal. Boat trimmed fine with it full.
When I had it remade, the guy at alloy metal works (Phil?) said based on the measurements he would call it 70 gal. It could have been made 3-4" longer but I wanted to be able to reach the bilge through the space behind the tank. So we left it the stock size.
--Kevin
