you say you check it to see if flat how did you check this ? did you look at the print out of the old head gasket on deck/heads to see low spots if any? reading old head gaskets will tell you alot. are the deck pins holding heads up .you would have a hard time over decking a sbc things like pistons would be coming out of the deck @TDC so it would have to be .025+ before you may see this .i would say the head chamber is crack or heads crack under a valve seats. heads need to be look at better