Of course, a sealant such as Bar’s Leaks is NOT the solution here. A sealant is also likely to plug up an old radiator, resulting in more serious overheating problems.
As I already pointed out, what happens on this engine is that the steel crossover pipe becomes corroded away in the area where the O-rings seat. This pipe is made up of two parts: one connects directly to the water pump with an O-ring, then the two parts connect to each other with O-rings. The second part connects to one of the radiator hoses.
By the time this pipe has enough rust to leak, it leaks because the O-ring(s) must seal to surface(s) which are pitted and out-of-round. No sealant can possibly fix that.
Replacing the crossover pipe is not much harder than changing the timing belt and water pump (which is fairly difficult on this engine), and should be done at the same time.