They can also be loosely grouped based on their block dimensions:
G1 includes the oldest Chrysler, DeSoto and Dodge hemis with rear-mount distributor. Although very similar in design they all have different deck heights and bore pitch. This series does not include any LA-based or Plymouth-branded engine.
G2 is based on the RB platform with front-mount distributor.
G3 is a mod on the LA/Magnum engine.

More data on my page here:
http://victorylibrary.com/mopar/m-table.htm