[HawgRyder]

Ed...by the look of the diagram...the R1 resistor is acting as a load for the input of the box.
I would check your tach output with a similar load to ground just to make sure the speed sensor box is not goint to load down the MSD box and cause a problem.
If the speed box does load down the MSD box....the ignition system may become intermittant at higher RPM's or overheat and fail.
You might find that putting in another resistor in series with the R1 (12K Ohm) will still work.
If it were me....I would stick a Pot ( variable resistor) in series with the input, and dial it up until the speed box stops working, then back it off until it's stable, and measure it and replace with the closest value resistor at that time.
This would still allow the speed box to work, and not load down the MSD box.
My $.02
Bob