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I felt a followup necessary.  Careful as I was testing the tach before installing it into the car things can always go wrong.  I identified all the correct wires, verified voltage at the connector, rang out the entire circuit and connected it with confidance.  Except it didn't work.  I'd start the engine the tach reading would jump up and show idle RPm and then shut itself off.  I'd restart the car and it would do the same thing.  I pulled the dashpod out and all was correct.  I felt I had damaged the adaptor playing with the connections and ordered another one, I installed it a couple days ago and it did the same thing, it'd read briefly and shut itself off.  I knew the tach had power because it's supply was from the water temp gauge and it was reading proper.
I pulled the pod out and checked voltage at the water temp post and it was reading crazy - I had forgotten all about the rectifier!  I pulled the dashpod completely out and sure enough I had followed the original Electrical Assembly Manual and fed all the gauges with rectified voltage and the tach don't like it all!
I split the circuit at the rectifier and fed the tach from there, reinstalled the dash pod and it works like a champ!
Don't you just love these small victories!

Got it.  The tach Red is +12v, the tach Green will connect to the Violet from the MSD adaptor.  On this harness that would be wire #31, White w/Red tracer, in other words its replaced the oil temp gauge on a normal Mustang.
The tach reads a little low RPM wise. 
The adapter box connects as if it is a voltage triggered tach as if the original trigger came from the Negative side of the coil. 
I got brave and connected it every way possible and the above is the only way it worked, on the bright side I didn't burn it up so theres' that.
Is there an available schematic for these cars?
Thanks for the interest and help, Oj

Why can't anything be simple?  I decided it was best to remove the dashpod and benchtest the signal rather than go thru a mess of work and not know where to troubleshoot if it didn't work.  I have the dashpod out and on my bench where I have a Sun504, I stuck a distributor in the machine, connected an MSD box and followed the instructions connecting the MSD 8920, it didn't work.  The problem might be the pigtails are swapped coming out of the tach, it has a Red wire and a Green wire.  I presumed the red to be +12v, the green to be tach signal, however, I see stamped into the tach mounting bracket adjacent to where the red wire exits it is stamped 'white red st' and adjacent to the green wire 'yellow black st' 
Can anybody confirm the Red wire is the 12v supply?
I tried continuity readings, Red to Ground, and got about 1 megaohm.  Geen to Ground is 0 ohms.
Again, what I have for schematics, and used, is the Mustang Electrical Assembly manual.
Thanks, Oj

Thanks for the responses, clarification and confirmations.  I feel a little more confidant connecting it now, I have the MSD #8920 to connect into the harness.  I'll ring out the circuit and wire the adaptor in this morning but can't fire the motor until I do some maintenance on the valve train. 
I'll report back with success or failure, Thanks again  Oj

That would be the +12v then, the tach would be a current triggered tach.  Thanks for that, I was guessing voltage triggered with the tach wire going to the ground side.
I appreciate you taking the time to help, Thanks
Oj

I'm working on a GT500 that has been fitted with MSD ignition, how did the original tach connect to the coil?  Was it connected to the positive or negative side of the coil?
Thanks, Oj