A 340 Tach question....

The factory harness should have a gray wire (with a black tracer ,I think) that goes from one tach terminal to the negative side of the coil. The other connection (blue wire)should go to the fuse box and supply power for the tach and light. The tach housing will ground to the dash. On my 68 the spade connector on the tach goes to a gray wire connected to the negative side of the coil .The terminal without a spade connector on the tach goes to a blue wire with a 90* push-on connector. Yours should do the same. HTH.