I was assembling the aluminum intake on my 340 this Winter and found that my cast thermostat housing leaked just after I put in the coolant and let it set overnight. So I took a file and cleaned up the bottom side of the cast housing, then reapplied a new Felpro gasket with the Permatex (waterpump/light blue) sealant. It leaked again after sitting overnight. So I took it off again and used another new Felpro gasket but this time I used the Right Stuff sealant on the mounting surface and the blue sealant on the bolt threads, and it has not leaked a drop.

Note: When I had the intake off for cleaning/paint, I lightly filed the thermostat mounting surface and found it to be fairly flat.

In all my years, I don't think I have EVER had a t stat housing that did not have a leak, no matter HOW much I cleaned and made sure things were flat, etc. . . . . went with this :

http://www.terrysautolab.com/

Have not had a single leak with this !! . . .Yay O ring seals !! . . . I am not too worried about "non stock" look, so for me this was THE SOLUTION !! . . .well worth the $$$

Good luck . . .

grey permatex