The HEI has a TACH terminal on the cap. Plug it in and it should work.
Some tachs require a "sender" which is a resister to smooth out the signal. The factory tach shouldn't need anything as it is already preset for a V8 motor that takes the RMS number of pulses from the coil using an electromagnet and two permanent magnets inside the tach (and a spring to pull the needle back). The faster the pulses from the coil the stronger the magnetic field generated. The electromagnet is balanced in the magnetic field from the two poles of a horseshoe permanent magnet. The spring pulls the needle to zero and the force of voltage pulses from the coil is converted into an increasing DC current which pushes the needle towards the red side of the tach gauge.
Where an HEI differs from points style distributors is the HEI generates amperage (current pulses at a very low fixed voltage). The HEI module takes this low voltage current pulse and converts it into an increasing voltage pulse to trigger the coil. A coil used with an HEI differs from a points coil in the number of primary windings as they differ in the primary voltage.
If you have an ignition box (such as the MSD 6AL) you use the TACH terminal on the ignition box and not the tach on the HEI cap. You also have to disconnect the HEI module inside the distributor and connect the green and purple wires from the ignition box to where the two terminals of the HEI module plugged into the magnetic coil of the Hall Effect sender (pick-up) of the distributor.