I've become a big fans of centrifugal superchargers, for virtually all applications. I know that the Supercharger Store has put one on a 455- http://www.thesuperchargerstore.com/Gregg%20Hurst.html
If you definintly want to go the roots direction, I think that Arizona Speed & Marine has done this before, but you'd have to contact them to make sure.
I assume this is an all-race application?
-Bob C.
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