I have the Dr. Gas unit with 3" B&B's. It's the cheapest, and they have done quite a bit of development work on it, (with Mustangs).
Painted mild steel, not the prettiest, but works good. I clocked 117mph. in the qtr. with a stock motor, Coplon duct, Watsons, Dr. Gas X pipe and 3" dumps, no mufflers.
One "problem". It is loud enough to WAKE THE DEAD !!