13/16" seems an odd shaft size to me as well. are you sure that's the diameter?
If so are you trying to stick with that and be "original" or have you considered an entire new shaft and joints from say 1"?

I'd look locally for truck equipment dealers/installers, they may have all the parts you need in stock and might be cheaper on the keyway cutting cost than a job shop machine shop.

Have a look in this catalog, they do list 13/16" components.
http://www2.dana.com/pdf/U330.pdf

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-