Well,
@gtgto, I had the same problem. No matter what I did, it still leaked (Edelbrock Air Gap on a 340). I tried several kinds of gaskets, a few housings, and trying to sand the surfaces flat, but it still leaked. Then I got a billet aluminum housing with an O ring from Billet Specialties. It worked right away and hasn't leaked for 7 years now.
Thermostat Housing - Chrysler Small / Big Block (billetspecialties.com)
Edit. The original post disappeared on my end, so I rewrote it with a more accurate link.
