The 392s have a 1/2" taller deck height than the low deck 331/354. As the deck height rises, the distance between the heads increases. Most manfactures usually just make a wider manifold to compensate.

Chrysler for some reason made the heads wider which means the low and tall deck intakes interchange. In order to put 331/354 heads such as the low deck 555 heads (usually listed as the the best flowing stock early hemi heads) requires a spacer.