thermostat housing leak

Those intakes have 2 sets of bolt holes and all 4 holes go into water. I think that is where the problem comes from. Plug the unused holes and use sealant on the bolt threads.
Many will overtighten the bolts which squeezes out all the sealant and compresses the gasket and doesn't give it a chance to swell and seal. Even with stock intake and water neck I will evenly tighten the bolts to something like 15 foot pounds ( guess ) and I have had a small seepage for a few days. It does stop leaking completely though.