We did the research on this a few years back. The single stud switch was used in conjuction with shifter mounted switches until May 15, 1968 (according to the parts books). At that time a running change was made to the three prong switch.
Now bear in mind that even though this is the "Final Version" of the parts book, the old "Never say Never" rule still applies... but from the "horse's mouth", 5/15 was the cut-over date.
That said, a single stud switch can be converted to 3 prong functionality by installing the switch, changing the manual valve detent lever, and routing the wire harness. The wiring harness is available from Bill and Rose Evans. The switches are widely available, and if you are unable to locate a used detent lever, new ones are available through transmission part suppliers.
I've done it on a few vehicles, nice Saturday project.
SC