steering coupler

I think it's the second one down on the page. GM/pinto...3/4" smooth on one end to weld to the steering shaft and 36 teeth on the steering box end. But check the number of teeth on your steering box to be sure!
You don't really need to weld them to the shaft. Borgeson p/n 013464 should be the one you want. I believe that double pinning it as shown on the Borgeson web site may actually be better. I know that when I put my Borgeson late GM p/s box in my '69 Dart that I pinned it as shown here. Works great: