Stop Leak might solve your problem for awhile but the drawback may be you stand the chance of eventualy causing your heater core to become cloged in spite of their claims. The better temporary repair is Alumiaseal or the powdered aluminumn sealer which dosen't tend to clog the heater as much. Best is to try and get by until the head can be removed and plan on having it re-surfaced. It is very possible that in overheating it has become slightly warped. The danger with continued driving is that it is already leaking to the outside and might well start leaking tward the inside and into either the oil gallery or the cylinder. When it is re-surfaced plan on doing a valve grind and then you might consider cc'ing the head which is simple. With the valves done and closed, place a piece of plexaglas over the combustion chamber with the head upside down and level, have a hole drilled in it. Take a 50cc hypodermic and fill each combustion chamber one at a time. Take note on how much each holds. Then with a dremel or grinder take small amounts out of the combustion chambers so they all match the largest one. Stay away from the valves and try to make the grinds small , spread out and smoothe so you don't disrupt air flow. This will make all cylinders have the same combustion area. This is done beacuse when re-surfacing if you take say .035 off the head surface to get it flat, you have reduced the chamber on each end by a slight amount. It all goes together to help make the engine run smoother by equilizing them all. In race engines they go a little further and match the deck height of the pistons ( from the top of the head surface of the block to the top of each piston.) All cylinders being equal goes a long way to balancing out the combustion or compression and in turn a smoother and better running engine just like balancing an engine helps take the pound out of the bearing surfaces and extends the life of an engine.
Good Luck and do it right once and save yourself headakes later.
Dennis