Pope is right, but not quite as simple as it sounds.

The intake and exhaust are different. The intake usually has a mismatch on the heat crossover. THere were alos cooling systems vevisions over the years so certain heads/head gaskets will not interchange.

THe big one is the pistons, the valve reliefs are located differently and poly/hemi pistons must be used with the appropriate heads.

The conversion is reasonably simple, but it does require careful selection of parts.

The Poly heasds were a pretty effecient design and although not as visually impressive as the hemi's they are a good engine in their own right.