You are not quite correct. Mach3 needs the index pulse to be at least 200 microseconds to reliably read it. That means that your speed can be as high as about 150,000RPM as far as Mach3 is concerned (considering that it takes 200 microseconds for the pulse high state and the same amount of time for the pulse low state).
The simplest way to go, and one that I used before I had a servo on my spindle and SS to read the encoder index, is to build a simple circuit based on the 555 IC. You can calculate the resistors you need to get a pulse width just above 200 microseconds. It had worked perfectly for me.
Daniel