U Joint Cap Size

Going to do the U Joints on my 71 Swinger with a slant six, automatic trans.

Was looking up the joints and there are two listed. One has a 1.126" Cup and one has a 1.078" Cup.

Is there any way of knowing what size I need before I drop the shaft and check one? Any method to the madness?

Will the rear be the same as the front since I can measure it once I unbolt the strap?

Many thanks!

Brian