I had a persistent leak there, too.
Finally I got it to stop leaking by mildly scuffing up the aluminum surface with some fine autobody sandpaper. Then I used a Spectre Industries brand 2 piece Thermostat Housing with an O-Ring gasket (dry-no sealers or such.)
Yup. Just the O-Ring and nothing else.
Joel Adams
C3VR Lifetime Member #56
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"