'67 GTS, 383 w/factory manifolds-What exhaust?

If engine is fairly stock I don't think you'll gain anything noticeable with an X or H pipe other than sound maybe. Factory pipe did not have this. I would go 2 1/4 " out the back magnaflow mufflers. They sound good but not to loud and turn down out the back was factory. Probably cost around 350 to 500 to have made complete.