To the best of my knowledge all Mustang tanks in 1968 held 16 gallons. The original floats were made of brass, and prone to deteriation, its amazing yours has lasted this long. You can get a replacement plastic float for your original sending unit if you wish or buy a repo unit from the Mustang places for a '68. Its not a hard job to do, the worst part is siphoning the tank. You can get a kerosene pump with the squeeze bulb on the end, and some clear vinyl tubing from the local hardware store. This will set you up fine. The clear hose lets you know when the gas is flowing. Oh... and for gods sake don't use your mouth to siphon the tank, gas isn't very good to drink.