The dyno test merged the collectors into one huge muffler outside of the building. As far as manifolds are concerned the 340 ones are good, but they were good on 340ci. You are now displacing 408ci, so by increasing the displacement you are now asking the air pump(engine) to pump more air, but restricting the outlet by pinching it off. It doesn't seem like a good fit to me, unless it is required for a class like F.A.S.T.. Additionally, I would think your 408 will produce more heat than the 340 did, cast manifolds act as a big heat sink right at the head, you will get this heat away easier with headers.