The circuit on this page will work for most any tach where your going from a 4 to an 8 cyl engine, so I'll leave it up here.

The input is from the POINT Side between the coil and the distributor.The output goes into the tach where the old input was. On my 75, it's a white wire with a black tracer.

Assembly doesn't have to be real careful. Make sure nothing touches other things, I put mine in a little plastic box about 2"x3"x1" inch. Nothing in the circuit get even warm, so you can use a closed box. I used a small piece of perf board and just soldered everything in. I did use sockets for the 2 chips, but the tip120 is directly soldered in.

If you hook it up backwards, you'll blow the tip120 and maybe the 4013. I did it, that's how I know. Just replace them and hook it up correctly, nothing in the car gets hurt. Make sure you put it on a FUSED 12v supply, never hook anything up directly. That's where burnt up mgb's come from.

How it works - The input goes through the 1k and the 1uf cap and 12v zener act as a frequency filter and voltage regulator/limiter. The signal coming in ranges from 200V to -25V. This would burn out the chips, so with this the signal is converted to a 12V   pulse. Mine runs about 16.5ms wide.

The CD4011 acts as a buffer and cleaner. It  removes most of the noise from the signal and outputs a 10V pulse that drives the CD4013. Thats a Flip Flop that is configured to perform a divide by 2 circuit. On the output it's turned into a 32-34 ms pulse, twice the input length. Because the cmos chips can't drive the tach directly, the TIP120 (darlington PNP's) is used to drive the tach. The (3) 2.2K resistors could be replaced with something close to a 700ohm 1/2 watt, but I didn't have one.

The values on the output aren't  critical. The 2.2k's are critical either, I just had a lot of them available.

Also - the led and 2.2k at the output are optional, they just show you the thing is working, but don't add anything to the circuit. The 1uf capacitor at the front end is very important. It affects the frequency response of the buffer circuit - if the value is too small, you'll get double triggering, to big and  it won't trigger all the time. I think you could use 1-2uf without problem.