1989 Plymouth Reliant -- Use AlumnaSeal to fix headgasket?

After my mechanic found the Antifreeze is leaking out of the headgasket, I thought I might put AlumnaSeal stopleak through the system to possibly avoid replacing it. One mechanic told me I could try this before I new where the leak is. However this mechanic was very adament that it may damage the radiator (which is new) and the heater core. Should I use the AlumnaSeal first or get a new headgasket (I'd rather not for this car).