I have built tanks for several vehicles and used 18g cold rolled which has to be coated to prevent rust. The factory tanks use what some call "leaded" steel; something I can't swear to. If you have a TIG machine I would consider aluminum or stainless. The 1/2 ton AD tanks are 13x13x23, don't know about a bigger truck.I have a step by step photo shoot of building one if you would be interested.


Evan