If you do use the MSD adapter with their scheme you might insert a potentiometer in between the 8920 and the tach.  That will allow you to control current through the tach.  As it stands the adapter might just be a wide open low resistance path to ground.  Who knows.  

Add the pot., and just open it up until the tach starts working.  Wouldn't be a surprise either if you put the old ballast there and it worked.

The adapter wiring looks a lot like ignition module wiring.

A followup on my last comment.  I think that you could use a $20 GM HEI module and that 12 volt square wave on the Gray wire, with the old ballast resistor to modulate current and get the tach to work.  That 8920 adapter has to be working just like an ignition module.  It triggers from the Gray wire square wave, and lets current flow to ground just like an ignition module.  

The HEI module is often used in place of the 280ZX turbo ignitor, which is triggered by a square wave.  At least you know that it passes ~4 amps, has current control, and won't short.  It's a known quantity, unlike the mysterious 8920 adapter.

 

Sorry Parkstos, your head is probably about to blow up.  One thing you could do is wire it up using just the ZX module.  The MSD box can be added later after you get more information.  It's an add-on, for what you have, not a requirement.  The engine will run very well with just the ZX distributor.  The MSD box advantages are more for low RPM, somewhat "dirty" engines, like with a big cam and carbs, that tend to load up the plugs.  You don't really need the MSD box..