I would suggest tracking down the problem and repairing it rather than putting stop leak in. Stop leak can temporarily remedy a small leak. But, it can also clog small orifices in the cooling system. Think about it, if it's meant to clog/stop up a hole, it's not really a good thing to introduce into the cooling system. Just sayin'...

This is a bad idea, period. I would not do it. I've done it before, and regretted it every time. In the case of a Montero, it tends to fully clog some of the radiator flues that were only partially clogged before, making the cause of the blown head gasket worse. I'm convinced that most of our trucks blow head gaskets because the radiator isn't cooling any more because of blocked flues. Any overheat has the possiblility of eating the head gasket, because the head bows up and stretches the head bolts and kills the gasket clamping force, and the combustion pressure and cooling system pressurization combine to kill the gasket seal.