You will have a bad exhaust leak. Air from the smog pump enters the heads through those holes and eventually into the exhaust ports to provide oxygen to burn unburned fuel leftover from the combustion process.
If you still have the tube that went across to the two heads. Cut both ends off and flip them side to side and install with the hole face out. Use new gaskets.
Most speed shops can get you a special plug that is actually threaded on the inside so that you can use the hole for something.....like an extra ground strap. I don't know how else to describe the plugs...they are common.