The most significant differences in OEM heads is the difference between naturally-aspirated (L36/L26) heads and supercharged (L67/L32) heads.

Supercharged heads have injector ports located in the intake runners inside the head itself. However, naturally-aspirated heads have no injector ports in the heads because they are located in the intake manifold for these heads.
Because of this difference both will have different valve covers as well to make room for the injectors.

L67 heads could be used in conjunction with the L36 intake manifold to allow the use of an extra set of injectors for a very high end build if necessary. This would be difficult to tune with the OEM PCM though.

Series 3 and series 2 heads have some small differences, the most noteable is the difference in valve sizes.

Series 3 heads use a 1.83“ diameter intake valve where Series 2 heads use a 1.80” intake valve.

Aside from that most other differences are not noticeable. Some people have said that the castings are cleaner on series 3 heads, however others have said there is no difference.

There was a change in the casting used in the series 2 heads sometime around 1997/1998. Eariler heads are known for having a tendency to crack in the combustion chamber. Later heads seem to have resolved these issues though.

Earlier L36 castings were labeled “2261” and later castings were labeled “4781”

This image shows a cutaway of the early 2261 castings:

This image shows a cutaway of the later 4781 castings:

Here is a cutout of the series 2 head cross section posted from Intense-Racing.com