[Mr 5 0]

I don't know the specific Texas motor vehicle law that might apply to headers but as long as your Mustang's undercarriage exhaust is intact (not too loud or leaking) I seriously doubt an inspector would recognize your (aftermarket) longtube headers as 'non-stock'. Just don't go for inspection with a roaring-loud exhaust (that attracts attention to your exhaust system) and you'll probably be fine.