Beacuse the hole in the early intake (and gasket) is larger than the hole in 72 and newer heads, 
the exhaust is able to jump over the gasket and into the dead hole above the crossover.

Either fill in the 'blind hole' above the exhaust crossover on the 72 - newer head 
or use a stainless steel block off plate. Using the plate will make the crossover 
not able to heat the under side of the intake manifold. Cold drivability may be affected.