I had issues with lower RPMs so I had to find the problem and modify the design. At first I thought the issue was that the voltage regulator was dropping out as the tach worked great at higher RPMs but acted eradically at lower RPMs when various loads where turned on such as the heater.
When I investigated I determined that the DIST input signal didn't have an appropriate return path to ground. Because the circuit is a negative ground design and the truck is positive ground the signal return path actually was through the voltage regulator. Not good. So I had to electrically isolate the DIST signal return path from the circuit and a pulse transformer was the ideal solution. The transformer also allows easy polarity reversal.
Here is a .pdf tothe modified design: https://dl.dropboxusercontent.com/u/13355828/STT/Tach/Layout2.pdf
I purchased a Sun Tach and controller off ebay and decided to upgrade the antiquated technology inside the controller. This included gutting the controller and adding a circuit board that mimics the old relay/Mercury cell design using a 555 timer IC. The timer IC is triggered by the distributor and generates a consistent voltage and pulse duration. The meter then integrates these pulses to display pulses per minute which correlates to RPM.
Here is a link to a .pdf of the design: http://dl.dropboxusercontent.com/u/13355828/Tach/Layout1.pdf
This design is based on this I found on the Interwebs: http://myplace.frontier.com/~wgmumaw/Sun%20Tach/Sun%20Tach.html
And a video showing the results:
I suspect that at the higher RPMs in the test the drill runs a bit slower because of the added load. Of course I have no way of knowing how far off the drill is since I don't have tachometer to test the speed. I have yet to install the controller and head but I don't expect any problems.