Another possibility - mill appropriate slots into either the harmonic balancer or the flywheel edge, mount an HO CPS at the proper airgap spacing above it, install a basic converter that takes 0/5 volt pulses and converts them to 0/12 volt pulses, and feed a RENIX tach with them. The RENIX tach is driven by the low side of the ignition coil primary wiring which is driven by the ICM output IIRC, so basically it just counts 0/12 volt pulses, one for each cylinder. I'm reasonably certain you could build the signal converter with 1/4 of an LM339, a pull-up resistor, and two other resistors to set the "trip" level. Mill 4 (or 6, if the cluster you are using is from a 6cyl jeep) equally spaced slots in the edge of the HB or flywheel, mount the CPS, feed it 5 volts (if you don't have a 5 volt source in the truck, a 7805 regulator and two 0.1uF ceramic caps plus a 1uF tantalum or aluminum electrolytic will do this for you) and then feed the signal to the converter, then wire the output to the input of the tach.