Magnum Heads....

From everyone I've talked to it seems to be the head castings that are the problem. The area in the combustion chamber is thin and the crack when thay get a little too warm. The engine doesn't have to overheat for this to happen either. As long as they were pressure tested after the new seats were installed and you keep an eye on the water levels, I'd suggest running a 160 deg. thermostat also, they should be good to go. I'm no expert by any means but all my research seems to point in this direction, hope it helps.