91 Probe LX overheating???

After about 7-8 minuntes of high rpms to the engine it starts 2 over heat. It all started to happen after i replaced my coolant. I also had a leak in the radiator so I bought some Bars aluminum and that sealed the leak but it still over heats. When I turn on the heater, only cool air comes out, not hot. Also my back middle spark plug starts to smoke after a couple of min of running. I took out the spark plug and wiped it clean and the well itself to make sure there was no oil and it still smokes. One person said my radiator might be out for the overheating. Any ideas on both problems?

well the smoke coming from a spark plug socket is a sign of a blown headgasket... as for the overheating with cold air coming from the heater id suggest changing the thermostat... quickie question onto why you are running your engine at such a high rpm that long? some advice that it its not good for any motor, and it could also be the reason why you are overheating and smoke coming from that sparkplug