Hmmm that is a tough one.. and I think I follow..
I assume you have looked around, but I think that a Downpipe from a company called HKS may fit your bill.
Here are some pics from one of the members from the NASIOC board.
http://northwoodsinsulation.com/Rall...s/DSC01212.JPG
http://northwoodsinsulation.com/Rall...s/DSC01213.JPG
It will work fine though it is not as efficient as some other designs as some people say and may allow you to connect your piping directly to it.
Comes I think in 3.0 and 3.5.
It is non-belmouth design so not as efficient. People say that <400 HP should be ok..
Hope that helps..
ALex