There is NO thermostat in at this time.
That is the problem, not the stop leak. The thermostat acts as a flow regulator in addition to temp. Without it, the coolant is flowing too fast through the system, not allowing it dissapate the heat properly. You need to install a thermostate, or at least a 3/8" id fender washer (old Flathead trick) to act as a flow regulator.
