I have been rebuilding a 292 Y Block.  Once the intake manifold is put on there are two round holes left exposed as well as 2 bolt hole.  One each side.  I have 2 other intake manifolds and they are the same.  The spare engine I have had a temp sending unit in it and the other side looked like a factory plug was installed, like a small freeze out.  Am I missing something?  Is there a crossover pipe.  The passenger side would have to be small as not to interfere with the distributor.