Just dump this stuff in the radiator and drive it for 20 mins. leak fixed and never coming back. jb weld may work as well.
You are casuing a complex problem to fix a simple one. get it sealed up or replaced. they are cheap enough to buy a new radiator if need be. Dont waste your money buying the wrong parts when you can get the issue fixed and the thermostat repalced for that money with some stop leak.