Im a little impatient to wait till wendsday for a reply from MSD on this issue. I finally put my freshly rebuild 2056 EFI back into my car and working on connecting all the hoses & lines. When I noticed that I have 2 (- & +) tach wires. I never noticed that before and now that Im adding an MSD 6A sys I dont know how to connect. It only has 1 gray wire that goes to the tach, but it doesnt identify how to hook it up to a 2 wire tach application.
Any advise would be highly appreciated.

I've never heard of a 2 wire tach. If I were to guess... The + is 12v switched and the - is the tach lead from the MSD.

Just looked at my car. the white wire from the tach adapter goes to the OE tach wire and the red wire from the tach adapter goes to the red (12V switched) wire on the MSD. You can download wiring diagrams from MSD's website.

After reading my post, I can't even understand it myself.
I guess that's what happens when you write a book from a smart phone.

So here's what I got:
1 negative (-)
1 Positive (+)
Wires laying underneath the right side intakes to the right of the TB. These are part of the engine somehow. There're marked for TACH - +.

My MSD 6A has 1 gray wire marked for tach. I have no Tach adaptor (that may be the problem)

Unfortunately I didn't take a picture nor am I near the car till middle of next week to give you more info. I'm just trying to get some idea in my head on how this MSD connects. Worst case senario, I'm wanting to know where the gray MSD wire connects to car if I didn't have a tach adaptor. I was once told by MSD that I "might need an adaptor, only depends on the tach". Is this the case? Does it make a difference if I have a Pertronix? Cause I do, but I don't remember those wire being part of the kit. I have accounted the pair of Pertronix wires coming from the dizzy and I know where does connect to. Its just the tach thats driving me crazy.

If you have a factory tach you must get a tach adapter.

I do have an original tach. So original tachs are 2 wire?

My tach has three wires that attached to the bottom row on the rear of the tach: red/wh for 12v, black/purp that comes from the coil, and a brown wire to ground

The MSD 6A requires their tach adapter. Be careful, they sell two, only one of them works on the 914. I use the 8910, NOT the 8920

MSD has an excellent troubleshooting forum, I get replys from the techs within 24 hours...

Worth trying just a normal relay
Used the same in my car to use the standard tachometer with the ecu I have. Same output
Much cheaper and smaller than the msd version