This can be maddening! A shop I worked at had a Camaro with a small block loosing coolant. Engine ran well enough, but lost coolant. Shop owner replaced the short block, had the valves ground and heads surfaced, still leaked. Finally found a crack inside a port. This was back when Chev small block heads were cast 3# lighter in Mexico. Blame the engineering not the foundry. Machine shop shared the cost, but no one won. Pressure test the assembled engine and then the heads and you will find it!