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The revs indicated on the tach are several thou lower than they should be. I swapped in a different tach and and still have the same problem. Could there be an issue with the connection at the trans or wherever the tach gets it's reading from?....or is it always the tach itself that goes bad?

The rev counter uses the signal on the coil (black wire). That can't be wrong unless you have weird ignition issues.
Which makes me think, is the ignition stock?

I should clarify...the revs are way off as to what they should actually be ie. the engine is idling at 1200 rpm(I have to bring that down but that's another issue) and yet the tach hasn't started to register yet!....eventually it moves but way lower than where it's supposed to be.
Re the black wire at the coil....is there supposed to be only one black wire at the "-" side of the coil or two??
The electrical diagram in my manual is hopeless.....where does the black wire go besides the tach?

Two black wires, I believe. One goes to the points in the distributor, the other to the tach. That is, if you got points ignition.

Well I only have one black wire coming off the coil and the car is running so it must be for the points....unless that black wire splits into two at some point and goes to the Tach?
At the thick wire loom at the drivers side shock tower I see a a black wire that's disconnected....could this be the lead to the tach? If it is I don't see where it's supposed to connect? I could have knocked that off when I was changing out the clutch MC in that hell hole?

I have correct myself a bit after looking at my engine. There is one black wire from the coil, that goes to the distributor (points). On the distributor spade connection it splits to another black wire. That one goes in to the wire loom and finally ends up at the fuse box connector. My car has a carburettor so the loom is different, but still, there should be a wire going from the points to your tach.