I installed a Realgasket thermostat housing gasket on my cast iron 67 about 3 years ago and not a drop.
Caution. They are installed DRY as in NO sealers. And torked to (IIRC) 12 INCH pounds.
My cast iron intake is all pitted from 50+ years of leaking. And it has not leaked drop.
Also clean out the blind holes of debres.
