Correct Ball Joint still not fitting - What am i missing?

The difference between large and small is obvious, so that's not your problem. It is best to get the threads close to starting and rotate the ball joint LEFT as though you are removing it. You will feel when the joint drops on the first thread.

As for the part number of the arm, it's right there in the numbers you posted. One side ends in "2" and the other ends in "3".