Apologies for dragging up such an old thread but this was the only applicable one I could find via a search here and thought I'd share.
I plan on running a GPS speedometer as well, and was fully prepared to machine something up but on a whim image searched "datsun speedometer pinion delete plug" and found a match:
https://zcardepot.com/products/transmission-pinion-gear-plug-delete-racing-oem?srsltid=AfmBOoqKAdugm1tF7u0EpsepQLggtB1_N4C0VGzhN7Azlut4rc0esj0F
As well as here:
https://datnissparts.com/rwd-speedometer-drive-pinion-delete-plug-32707-aa000-replaces-nla-99996-c3040/
It's a Genuine Nissan part, p/n 32707-AA000 with a long compatibility list of boxes - can confirm fitment on a 71C tail housing.