Magnum exhaust manifolds

Non-Jeep magnum manifolds are basically the same. The early ones will accept a 2 1/4" down tube. Obviously the outlets are in a different location than your center dumps but if you're redoing the whole exhaust it really doesn't matter.

Will you notice a difference? Yes, but a lot of that will be from the dual exhaust. The magnum manifolds will support a mild 318 just fine, and when you decide to use that BBD for target practice, you'll have plenty of exhaust to support a 4 Bbl.