You should check to see if the holes have bottoms (Chev bolts go into the water jacket); if they do, you don't need sealant, but you do have a problem with head gasket sealing, which may be caused by schmutz on either gasket surface, incorrect or uneven torquing, or non-flat to warped heads and/or
block deck surfaces. Consult with a trusted machine shop, who for a fee can check all of those possible symptoms. Getting back to the head bolts, make sure their length and diameter is within specs, that they're not corrosion damaged, and that they're wire brushed clean before reinstallation. Dip them in non-synthetic engine oil, and torque strictly to the steps and pattern set
by the factory. By doing this, I've only experienced only one failure, caused
by a crack down in the bolt hole (on a 289). Good luck!