If you have not read the early posts of Derek Bell on his early quest for an improved UN-1, be sure to do so. His first attempt was a failure because he made the one piece shaft to hard. It was tough, according to specifications on the material but did not have the ability to deal with the twisting shock loads. As I recall he spelled out the material he finished up with.
For what it is worth, I have a Chris Cole modified and rebuilt box with the Quaife diff and it is great.
