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.