Reserve fuel tank?

well , during the gas shortage of 1970's my father and uncle where making reserve tanks and installing them as well . there was a chrysler electric switcher valve that switched fuel n vents , but sending units as well . i think it was for duel tanked trucks . they made a 11 gal tank that filled with the oem tank on an amc gremlin . and put a charge flip cap on the quarter panel top of a giant gm buick . but if the spare tire was under the package shelf and a gas tank was in the round , it would hold more fuel . trunk floor would be flat the spare could sit on the passager side of the trunk floor like , b and c body rags do .