T-400 speedo gear shaft too big

[Judgeyoung]

In fixing details on my recent 2-owner 72 GMC c2500 find, I pulled the speedo drive after the cable and speedo checked out good. Sure enough, the gear was twisted off, and either eaten up, or hidden in the tail shaft.

The drive gear appeared natural color, or maybe yellow which is usually 19 teeth. If my numbers are right, I need a 42 tooth for my all stock 3.54 rear and 235-85-16 tires. The remainder of the original shaft looked like it might have been white, or grey, but I am not sure.

Today my green 42 tooth gear came in and I pulled the drive. The new shaft will go in to where the spiral starts, but is too large to go farther. The old shaft mics around .312, as does the old piece. The new shaft is.331. I thought about reaming the hole with a valve guide reamer, but decided to slim the plastic shaft instead. I did, and it fit nicely, with the spiral remaining barely visible.

UNFORTUNATELY... when I went to re install, the drive would not go completely in. It appears that the bearing hole at the gear end of the shaft is either twisted off flush, or otherwise not opened to accept the end of the shaft.

Now, the questions:
1. Why did the first one shear?
2. Are my gear calculations in the ball park?
3. Why was the shaft too large?
4. What is up with the inner bearing hole?
5. If it it part of the old shaft, is there any way to remove it with the tail housing still on?

Thanks in advance for you help!