I have a 1987 250 CCP with the below deck fish box. I take 120lbs of ice on every trip. Usually 40 lbs in the 120qt cooler for drinks, 20lbs in my aft. cooler for food and the rest for the fish. I sometimes use the cooler ice at the end of the trip for the run back in. 60lbs should be your minimum IMHO.
JG