I replaced the sender on my '89 Spider about 3 months ago. I used a friend's mechanical gauge to check pressure - I belive the hole is 10mm. And I think the pitch is 1.0 mm (fine thread). I gave the gauge kit (Snap-On) back right after I was done, so I can't be sure right now. I don't think there's any difference with the older engines.
Take a 10mm open end wrench and fit it to the threads on the sending unit. If it's a close fit, it's a 10mm fitting. 1/8" NPT is a little smaller, I believe, and the thread pitch is more coarse.
[COLOR=Navy][FONT=Comic Sans MS]Geo.
89 Spider Graduate
60 Triumph TR3
73 Triumph GT6 Mk3
03 Mazda MPV dog hauler
91 Nissan King Cab parts hauler[/FONT][/COLOR]