Rust is... oxidation.
Fire is... really rapid oxidation. Even if you painted a burned truck immediately after the fire went out, there would still be rust under there unless you mechanically or chemically removed it.
Not to mention the warpage.
The 30's trucks relied heavily on wood as structural members in the cab. Restoring a twisted metal cab without the wood is an impossible task, you will have an easier task if you mine and smelt the iron ore yourself and then turn it into steel than starting with a twisted burned out hulk.
