Blocking the heat crossover, IN the head.

If you are running headers, or stock exhaust manifolds with the butterfly removed, then there will not be much flow going through the passage. The original exhaust manifolds had a butterfly valve in one side for a reason. It would be closed when cold and therefore create more restriction on the one side, so the exhaust gasses would be forced through the port under the manifold. As the engine heated up the bi-metal spring would open the butterfly valve. Then there should be little to no flow going under the intake.

I remember the intake manifold from a 1968 383 2 barrel, the passage was almost completely plugged with soot. That probably insulated it pretty good from heat.