I just finished reviewing the 75 edition of Ford Car Parts (aka MPC) for gas tank drain plugs. Remember, this is a "replacement" document. No drain plugs are shown in the illustration section for 1971 and up Mustangs, and other Ford products as well. There is also no mention of tanks without drain plugs on some 1969 models. That is not uncommon, the MPC lists service replacement parts, not Concourse or assembly line. The replacement drain plug listed for all 64 to 70 Mustangs is 353055-S (PP29) 1/4-28 X 3/4 extended head.
Jim