tach wiring

Ok I have a 71 that I put in an 81 motor with the 81 alternator. I need to wire the tach up. I have no idea if the tach worked prior. What do I need to do to get this to wire up.

To get this car running I cut the black/white wire and green wire that go to the coil and have spliced them together in the engine bay. I ran these two wires to a relay so I can get a full 12 volts to the coil instead of I think I measured 6 volts or something that it was prior going to the coil.

Any help please.