Search:

You can use either frame. The front engine plate needs to match the frame. If you have a 2A engine plate you need to fabricate an adapter to make it work in a 3A frame. Willys did provide an adapter...

I think you have hit the most important points. There are differences in the early vs the late 2A frames. When the laminate frames were replaced by the heavier gauge frames. The boxing should be...

jnj, here is a video that might help. It shows the CJ2A frame compared to the earlier MB frame. The "box in" should be there on all of them. The strip of metal does appear to be an add on.
...