I have a fair amount of data on this very subject.
We had a 1993 Ford Taurus GL, 3.8, that blew the heater core. 1 treatment of the copper Bars Stop leak, and it sealed it up. The issue never returned in the remaining 2 years the vehicle was in service.
My 1993 Park Avenue also blew a heater core. I had it replaced. The replacement failed within 6 months. An Aluminum Bars Stop leak treatment followed by a copper Bars Stop leak treatment held that together for another year until it failed again, and was replaced by a third heater core. The third heater core failed 1 year later. Bars Copper stop leak was attempted again, however it failed within a month. I was not open to the idea of paying another $300 for another heater core replacement, so I bypassed the heater core a few months ago. Since then I have noticed odd behavior from the cooling system, including a 210* operating temperature with the A/C running wide open. All other vehicles are usually running at thermostat temps, especially with the amount of fan that is on the nose of that car.
Today on the lunch run, the car literally boiled over when I parked. It was only 80-85*. I started the car back up and let it idle with the A/C off. I don't know how hot it actually was, but it was hot enough for the high speed fans to run for a solid 3 minutes (225+), but not enough to trip the idiot light (255*).
I realize this is a lot of words, but I offer this: Stop leak is a crap shoot at best whether or not it will actually address the leak. Which stop leak you use is a whole different crap shoot, as some work better than others in different situations. In the process, it clogs things up, whether or not it actually addresses the leak. If you wish to delay the repair, just bypass the heater core and do without it for the time being. No more leak, no more worries.