I don't recommend the Mr. Gasket chrome therm. housings. I had one on my for about 3 yrs. I noticed a slight leak one day, and found it was coming from the side! Apparently, antifreeze is corrosive to chromed pot metal.
I replaced it with a stock GM aluminum housing with only a gasket, no sealant.
ryans69chevy, remove the o-ring and place the housing on a perfectly flat surface and you'll see why it won't seal. The chroming process warps the metal. If you have access to some kind of disc or belt sander, you can flatten the surface to make it seal. Like I said above, though, you'll have to replace it in a couple years.