At our shop, we see a LOT of small block Chevrolet engines leak at the intake manifold. It seems to happen when Dex cool antifreeze is left in for more than 5 years without being flushed. It also could be that GM changed the gaskets about the same time they started using Dex cool, I don't know. However, we flush the systems, new antifreeze (any brand) and add Bars leak. When the leak stops, don't flush the system until it is due again. If the heater core plugs up, it had problems already and the stop leak only made it worse. In that case, reverse flush the heater core only. DON'T add more if the leak does not stop. One dose only between services.
Paul

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84 500 SEL (307,xxx miles)