Had a guy in Seattle rebuild my tach and modify it for use with an MSD. I ended up not using the MSD. The Tach doesn't work as of yet. When I replaced the under dash harness I had to use the wires off the old harness that fires the tach. Given instructions from the M&H tech I spliced the wires into the new harness. That may be my problem. Or, there may be a problem with the re-wire of the tach. Anyway I do have a schematic to direct wire the tach so that may be my next move.
You say you got an adapter with the tach you got? Not sure what that is. My tach is original to the car and it didn't have any sort of adapter. Can you shoot a pic of it?
I'll dig up the info on the guy who rebuilt my tach. There's also a company in Tennessee that knows how to rebuild those tachs.
Here's a pic of my rig. The plastic piece goes between the Tach and the console so you can adjust the angle. I still need to look for a wiring diagram to see how it all goes. Like I said, My car did not have this factory tach. Got it from one of my swap meet buddies at Carlisle. Looks darn near NOS to me. Also, I'm not sure if my console plate has the knock out for the tach. I'm gonna pull the dash and the console next week, so I'll see.