Welcome to CHR! The term Rat Rod brings up all kinds of ideas, but the bottom line for every build needs to be safety and reliability. Tell us what you want to end up with, and there's lots of folks to help you get there. Depending on how radical you're wanting to go you might start with a complete old truck of the era you're wanting, or you might start with a pile of 2x4 tube stock and fabricate from scratch.