I have 4 or 5 sets of steering linkages laying about.
All of the 67/68 P/S systems I have utilize 5/16-24.
My manual steering parts car (Mar 67) uses this size as well.
Except for the Competition Suspension (CS) P/S on the Shelby.
Those are 3/8-24, are marked TR with 3 radial lines and use a larger red lock nut.
The Shelby uses the same nut/bolts as a manual steering 65/66 steering linkage I also have.
Makes me wonder if Ford went bigger for the CS?
If you need 5/16-24 nuts or bolts, PM me.
[on edit s/7/5/]