Blocking the heat crossover, IN the head.

I've seen it where aluminum was melted and poured into the passage. Naturally you need to do it from the inside of the port and block the intake manifold side. I don't recall the details, but I believe that high quality aluminum needs to be used (Pistons were mentioned, but there are a couple of different forging materials) and I don't know what type of cleaning needs to be done for best adhesion. Propery filled and contoured, it's suppose to be good for a few cfm of airflow in that exhaust port. But it doesn't sound like an easy job.

I've always blocked the passage between the intake and head on my race engines with a thin piece of aluminum sheetmetal at the intake gasket and coated any exposed surface with hi-temp silicone. Never had a blow-thru on them. Don't know how long it would hold up on the street.