What is the best method to clean 68 gas tank
first of all, as everyone is saying, just get a new tank. the only exception to that would be if there is some historical significance to your car that would be harmed if the original gas tank is removed. Chances are, the tank isn't original anyway.
If you absolutely have to keep the tank, don't coat it. I've seen literally hundreds of gas tanks, motorcycle and auto, that had one of the popular, and expensive gas tank coating treatments done to it. Without exception, they ALL eventually fail, and begin to de-laminate from the metal and then you are much worse off than before. NO ONE is making a coating treatment that lasts, if they tell you differently, they are flat out lying.
The only treatment I use if a gas tank can't be replaced, usually on vintage motorcycles, is an electrolysis process that deposits a hard and durable phosphate coating on the inside of the tank that is only a few microns thick.
briefly, you fill up the tank with a solution of water and washing soda. get a battery charger and connect the negative clamp to the outside bare tank metal. Attach the positive clamp to a sacrificial anode, a ferrous rod, iron or steel, not stainless steel, and insert it into the tank into the liquid solution, making sure not to contact the metal of the tank.
turn on the battery charger and some nasty black vile sh*t will start bubbling out of the tank and in a few hours the rust will be gone and the phosphate coating will remain.
rinse out the tank with hot water and blow it 100% dry with a hair blow dryer.
The down side of this treatment is that it's very hard on battery chargers. In the process of restoring about 50 vintage motorcycle tanks, I went thru 4 NAPA chargers one summer at a vintage motorcycle repair / restoration shop where I worked in Austin. After the 4th charger died, the guys at NAPA refused to exchange the burnt out charger for a new one and gave me a refund.
They very politely asked me to go somewhere else for my battery charger business. I'm they are still wondering how I burned up their nice battery chargers.