I'm actually making some progress:
Got most of the hardware and programming worked out, finally! The tiny black thing above the resistor is a Hall Effect sensor which I think is used on our original tachs. Kind of cheezy, but I was thinking about just epoxying a little magnet to the PS pump pulley and mounting the sensor near it. It wouldn't be as accurate as a crank mounted sensor but an LED tach is more for looks anyway.
This way I could mount a big clock in the tach spot and still have some sort of RPM reference.
Thanks for the pic Gavin! I may put a thermometer in that spot and incorporate the LED tach into the speedo some way. Maybe mount the LED's in a circular pattern around the speedo dial? Too garish you think?
Any other ideas on mounting the tach LED's?
I already have 2 LED's mounted here:
And I don't think they look too out of place.