old 1980 wellcraft info

Heres the deal. I just got a 1980 180 fisherman, the guy let the inboard gas tank sit for a year or two when the gas gauge went out and put a 12gal tank under the console. Ive been flushing the tank since ive replace the gas gauge and found out the sending unit is bad.(gotta cut fiberglass to get to it) Does any one know how much fuel the inboard tank holds?