Thanks Neil, I do have a Buddy with that era manuals, I'll check it out. It may have just the info. And he also has a small pile of sending units, might find a good one there.
Believe it or not, I'm working on a digital Speedo, and planned on using the pickup right at the speedo drive on a BT tranny. I've fabricated a small cup with a notch, and figure on mounting a home built sending unit right on the speedo drive unit. Meanwhile I've started writing programming for a microchip to count revolutions and stuff to put up the output on an LCD display. It's a long way from finished, but it's another challenge. Haven't tried building a speedo before..