I'll tell you what I know...
The wheel house tops are 2" longer on a 2a, that translates into less leg room.
The 3a windshield frame is taller than the 2a, but I don't know by how much.
The M-38 has a larger opening for the fuel filler neck, and the floor pan over the transmission is removable on an M-38, much like the early CJ-5. Some M-38's have a larger/deeper fuel tank..
M-38 dash is very different-gauge location, switches and it has a glove box....2a and 3a are very similar..
I'm sure there are a multitude of other M-38 vs 3a differences..top bow fastening, grab handles, hinged grill, brush guards on the headlights etc
I don't know about frame mounts....