308 vs. x heads

From "how to hotrod small block mopars" by Larry Shepard P65.

"They introduced the swirl port 360 with the "308" casting number. Port-wise the "308" casting is the same as the newer "576". both have a similar combustion chamber to the earlier 340-360 heads. Both the "308" and "576" have 1.88" valves, the best exhaust ports and are the best production heads.

P76 (regarding pushrod holes)

"The 308/576 Head.

The '308' head was introduced on the 360 in about 1988 but was NOT exclusively on the 360. Other non-308 castings were also used in these years. So junkyards are not necessarily the best source for these parts. These heads feature the large diameter pushrod hole, to compensate for this the production engineers raised the intake port roof to get more air past the pushrod limit hole. The raised roof offers more horsepower potential. The exhaust port also has a W2 style short side radius and improved shape for W2 type flow"