I think a lot of us want just straight pipes, no fittings. Should cost like two bucks I already have a home depot reducer so stepping down from 3" is kinda unnecessary, just adds complexity. I think what we all want is a simple piece of mandrel bent 3" piping that places the filter in a decent spot, close to stock.

[edit] Sorry, I just noticed your reason for necking down. That kinda sucks, but what can you do...