[Peter Serio]

The hood tach as a reproduction part has been made by over 1/2 dozen different suppliers going back over a time span of at least the past 25 or 30 years. You do not say nor is anything labeled or marked as far as how long your old tach has been on the car or where it came from.

At this point there likely is no way to determine that anyway. Each supplier had their own way of making them as far as the circuit board and meter construction goes and none of the new ones are anything like the ones from even as "new" as 10 years ago. That said the modern one that I think you have now goes thru a "boot startup" every time you turn on your key. Your new tach needs at least 3 or perhaps 4 wires of electrical contact to work. (Some reproduction hood tachs are wired to be "lights on" at all times) where as the Some of the older reproductions had a separate grey light wire for night time driving. The original GM tach also had a separate grey wire.

Your new tach needs 12 volts of clean battery power, via the fuse block; thru a one or one & 1/2 amp fuse, when the key is turned on. It also needs ground. Last we need a wire from the new tach directly to the RPM signal output wire on the side of your red MSD6A module. I would NOT WIRE YOUR RPM seeker wire to the "-" terminal of the coil. I do not believe that you need any type of band-aid or adapter device with this new tach. It is made to work off of most any ignition system as long as it it correctly wired. Go to the MSD website and download the instructions for how to wire an accessory tach onto your MSD module.

It's pretty simple.

NOTE: The wire that is looking for the RPM signal from your engine needs to be fused. Same as the power wire. I recommend a one or one & 1/2 amp in-line glass fuse. It needs to be in one of those spring loaded plastic twist-holders made as part of the wire.

One thing about these new tachometer circuit boards is that they are not built to be very "forgiving." If you wire one incorrectly, or you wire one up without a ground, or you omit to use fuses in the power wire and the RPM seeker wire, the circuit board could be damaged. As far as I know none of the modern reproduction tachometers are made to be field serviceable. In other words they either work or they do not. There are no circuit diagrams available on any of that kind of stuff.
Be warned; if you cook one of those you get to buy another tach.