i have got a mystery leak on my 3.5, but i am pretty sure it is from the back of the valley gasket, as there is a green trail of antifreeze running down the bellhousing, i will replace it when the weather dries up for long enough, until then a daily check of the water level does me.
i have heard of k-seal before now, there are others out there too, wonder weld, steel seal etc etc, IMHO they are all just temporary cures to the underlying problem.