Thomas, why do you want to fit tubular exhaust manifolds to a 411?
Peter, the Chrysler 400 CID engines were fitted with "smog" heads to comply with new emission standards (they didn't breathe well) and the engine had very low compression ratio.
You could get an immediate improvement by fitting earlier (pre smog) "906" casting heads as used on the 383 - these are indistinguishable from the heads on your 412.
People often get them ported and polished and increase the valve size, but that all adds to the cost to the point where you could just get some after market alloy heads (which will look different).
There is a lot more you can do to increase the performance of these engines such as changing pistons and cam shaft.
You may need to get hardened valve seats in the 906 heads.
Just google "mopar 906 heads"
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