John, forgot to do it earlier but wanted to thank you for taking the time to pull two guns apart.
Regarding the firing pins, it would be interesting to know if a 16ga was built like a 12 or 20. Also thanks for the idea on casting. Seems like a good approach. Probably would need to clean up the plug first to remove as much surface irregularities as possible. Figuring out the firing pin will be interesting. I've got some concerns about it falling out too far when the hammer is cocked. I'll put together a sketch of an idea I have.
Attached are a couple of pics of my stock. Compared to your 12ga stock, there isn't much material to hold it together.


Here's another picture that shows a difference between the two gauges (12ga on top).

Finally, as for the cocking rod, I'd be interesting in hearing your idea. The rod is the same diameter and the length seems to be correct but might need a slight length adjustment. The picture shows my idea of how to modify the rod. Thinking that I could remove the necessary material and pay someone to TIG weld it. I'd be concerned about changing the hardness of the tip though.

Thanks for your help. It's much appreciated.
Kirk