<VV> U-Joint bolt questions

[Grant Young]

I am in the process of swapping a complete rear suspension between two 1965 convertibles. One with all new bushings, shocks, and brakes out of a rust bucket is replacing a badly worn set up in a otherwise very nice car. But, I notice that the outer u-joints are secured to the wheels of the cars with different size bolts. The donor car (with 13" wheels) has longer, larger castle head style bolts and the receiving car (with 14" wheels) has the smaller 1/4-20 bolts like on the inner joints. Two questions then:
1. When was the change made and why would the smaller wheels have the larger bolts?
2. What is the thread size on the larger bolts? I need to run a tap through the threads in the wheel to clean up some corrosion and dirt in them. (I know my thread gauge is right where I left it....?)
Thanks for any insight on this trivial pursuit.
Grant