Sealant for thermostat housing

[tucsonjwt]

I just replaced the thermostat on my 454 and used Permatex Hylomar to seal both sides of the standard paper gasket. It leaks (seeps coolant) all around. The water outlet (thermostat housing) and mating machined intake manifold surface were clean with little or no pitting. The water outlet was replaced about 3 years ago. I thought Hylomar was the "good stuff", but apparently the Permatex version, though expensive ($9 small tube) is not effective.
So, what is best to seal that gasket? Should I use the expensive thick Felpro gasket with built in O ring? If I used the Felpro gasket I will have a lot of cleaning to do first, and I am a little skeptical about getting a good seal on these old surfaces with just a thin O ring (Felpro says to install it dry), since there is some slight pitting in some areas, even though I did sand everything smooth prior to assembly.