How to find RPM from the square wave of a MSD box
Your tachometer signal is actually fairly clean and well defined, It would appear you are using the grey tach output wire.

The normal calibration for tachometers is 1 pulse per 2 cylinders fired:

4 Cylinder = 100Hz @ 3000RPM

6 Cylinder = 150Hz @ 3000RPM

8 Cylinder = 200Hz @ 3000RPM

Try measuring the frequency and see if this lines up, it would seem odd for them to not align with this as it would mean a large number of older cars that cannot easily be calibrated would not work with this system out of the box.

Edit: Based on jsotola's counting, it does appear it lines up perfectly to the V6 calibration. so 150Hz at 3000RPM, so for your system, use something like a schmitt trigger, feed it to your micro (probably its timer) and either count direct, or if you want really fast updates count the time between pulses (reciprocal counting)