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It was actually two stock 18-gallon Camaro tanks, one in the stock location and the other upside down in the trunk connected by a hose. They cancelled this option before the '67 Z-28 entered production by Ron Ogilvie of Bill Thomas Race Cars remembers doing these set-ups at the BTRC shop before the cancellation. My gold #1 car ran this double gas tank set-up but the team configured it on their own.
I guess you could say the rules were "evolving" in those early years Fred. At the Green Valley Trans-Am in April '67, the tech inspectors felt that the fuel tank capacity was too large in Fisher's car so they told him he had to reduce it. What the team did to reduce the capacity was put plastic balls in the tank. The plastic melted during the race and gummed up the whole system so he ended up not finishing.