"My overdrive donor is a 1971 P1800E. The overdrive switch is some sort of stalk-thing on the right side of the column. I'm not sure that it could be installed onto the 122's steering column."

It would probably be easier than adding a turn signal switch! I think it just screws into a bracket under the cover, you could probably fab up some sheet metal to mount it and then just cut an oval hole in the cover?

Another option is to take out the cigarette lighter (if you don't smoke) and install a matching switch there. that's what I did with my PV. Of course, the bezel says 'fan' and there are a few extra clicks (in all the way is off, all the other outward clicks are on) but it works just fine. I didn't use a relay, just wired fused 12V+ to the switch, from there to the 4th gear switch on the top of the trans, then to the OD solenoid. I used slightly heavier gauge wiring, but I don't think it pulls a bunch of current. Of course, using something other than a self-canceling relay like a toggle switch or a headlight relay can and does lead to the occasional 3rd-to-5th shift.
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I'm JohnMc, and I approved this message.