contact MSD the tach wire might be a buffered signal suitable for coil attached tachs
right on their (well Jeg's) website
"RPM LIMIT AND TACHOMETER INFORMATION
Tach Signal: The Ready-to-Run Distributor features a Gray Tach Output wire which provides a clean signal for most tachometers and even some aftermarket fuel injection systems. The signal output is a 12 volt square wave, 20° duty cycle. This wire is also responsible for programming the built-in rev limiter.
Rev Limiter: The Ready-to-Run Distributor has a built-in rev limit that can easily be adjusted from 2,000 rpm to over 10,000 rpm. The default is 10,000 rpm. To set the rev limiter, run the engine to half the desired rpm then ground the Gray tach wire (a jumper is supplied) for approximately one second. Every time the key is turned to the On position, the tach will display the programmed rpm limit. See page 8 for the programming procedure."
as I expected the tach wire provides the same signal as a points / standard electronic ignition coil signal
MSD Ignition 8388: Pro-Billet Ready-to-Run Distributor Chrysler 318-360 | JEGS
Also if you are just using the MSD ready to run Dist and not an MSD box you basically have a point ignition but using an electronic switch (no multiple spark.)
