I believe that the car with the 100 gallon fuel tank in 1964 was driven by Parnelli Jones. It didn't carry 100 gallons during the race because of a change in strategy by Jones' pit crew after Clark was so fast during the month and on Carb Day. The decision was made to carry only about 50 gallons or so to make the car lighter and hopefully a lot faster. Jones would have to make more stops, but if something happened to Clark(or Marshman, or both), then they could go back to their old 100 gallon strategy and still be competitive.
It almost worked...the problem on Jones' first pit stop was that the fuel tank was full of fumes because it was only half full of fuel to begin with, and as the pressurized methanol rushed into the tank, the fumes were rapidly displaced; and a spark from the refueling nozzle coming in contact with the receptacle of the tank caused an explosion.
Parnelli mentioned this in his autobio, "Parnelli: A Story Of Auto Racing".
He also said that he and his crew also seriously considered running on gasoline in 1964...with the same tank and strategy. Ponder what could have happened if they had...