The damage is from hot exhaust directly under the intake manifold. The 'heat-riser' used the exhaust gas to directly warm-up the intake manifold. It was intended to 'atomize' the fuel charge, during cold-starts, which means that the exhaust manifold had a mechanical flap, controlled by a bimetal spring to control the heat. Too much complexity for its day.
Now, you can buy a thin solid plate to go in place of the gasket between the stock exhaust manifold and the intake manifold, to accomplish some the thermal exchange without the direct exposure of exhaust gasses on the aluminum intake manifold. I suppose by skipping the exhaust manifold with headers, I'll probably have more fuel contamination of the crankcase oil, and a bit worse gas mileage.
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