Thrrmostat housing
Thermostat Housing Gasket
follow the directions
EXACTLY. Not "that feels about right" you will need an
inch lb torque wrench
also be sure the bolt holes are FULLY cleaned out and test fit your bolts in to be sure they are not bottoming out.
I'm going on 4 years with the stock 1967 CI 2bbl 273 intake (lots of pitting on the t stat side of the manifold) and a repo stock t stat cover.
NOT A DROP!
EDIT: I assumed you were working on a V8