Good question,

I'm not sure about TX but in CA, you have to go see the smog referee, and if he signs off on it, re register the truck as a custom built truck with a new registration showing the changes. The catch for me is that CA only allows 500 custom cars / trucks a year. When my reg in due in Feb of next year, if I don't need a smog check, I'll hold off on the new reg paperwork and do it the next year. Just look it up on the TX DMV website and see what it says...