63 GMC fuel tank capacity?

[68C15]

I have a 65 Johnson tri-hull boat. the factory tank up in the closed bow has 7 year old gas so I decided to replace it. as luck would have it we had a 63 GMC at work that was abandoned and it's fuel tank is identical to my boat's.

for my gas-oil mix ratio I need to know how much the tank holds.

FWIW the factory original windshield is from a 56 Chevy sedan and the horn is from a late 40's Plymouth.

[jonzcustomshop]

I think all 60-66 chev/gmc in cab tanks sold be the same.
that said, my 64 chev owners manual lists gallons,18.5 std, and 20.5 optional.
the std tank is about a foot short of the passenger side of the cab, and the optional tank almost touches the inside of the cab on the passenger side, like the drivers side does.