I have a 72 gmc truck that has the gas tank in the cab,I'm taking out that gas tank and putting a gas tank on the frame rails.I picked up a fuel tank from a friend that is rusted on the outside so can anyone tell me the best way to clean the inside of a gas tank

Muriatic Acid

Also, if it's rusty inside, there are products you can use to line it. You pour in the fluid and it coats the interior of the tank (you shake it and roll the tank) and it sets, so you don't get flakes of rust going into the fuel lines.

Best way is to take tank to a radiator shop and have it boiled out and a reputable shop can also pressure test it for pin hole leaks.