I can't quite make out the 1979 specs but the 1978 and 1980 Cuddy list a 46 gallon tank. The Fisherman's of those years list a 60 gallon, like Squids said.
I'd expose the top of the tank, take measurements and see what size you may be comfortable with.
Moeller has many to choose from, look at these in the 50 to 75 gallon range.
Pick the one of interest, click on "description" and then "diagram" for real dimensions.
http://moellermarine.com/product-cat...ks/51-75-gals/
Shop places like Jamestown Distributors, West Marine, iBoats and even Amazon.
Or simply do a search, for instance with the 60 gallon tank. "Moeller 032560".