[GMCPaul]

Then you'll need to cut the terminal end off the current coil wire and get a connector for the HEL distributor tach connection. Be aware

In regards to HEI use a stock style replacement tachometers they are already compatible UNLESS you are using some of the cheaper made Chinese HEI control modules. The real underlying problem with the tachometer not working in these trucks with HEI ignition is the output of the module from the HEI. The tachometer signal can be distorted either by peak size (causing "bouncing readings at idle and then smooth's out around 3000rpms) or funky square waves of various sizes- signals that do all kinds of things to your tach readings, to very low fat rectangle waves that don't do hardly anything. It's the modules design and output that is the problem.

Many of the inexpensive HEIs and/or replacement modules will cause these problems to happen, as well as these modules are NOT designed for performance use period. In our experiments with several modules the cheaply made and older type will severely reduce voltage output at around 4500rpms too! So, it's not only the distributor...it's the module and coil too that play a part in total performance and certainly tachometer compatibility.

Most "performance" HEIs have it all....great coil/module output to 6500 rpms and complete tach compatibility. Plus, if you have a performance coil, don't just put a standard module in...you can burn it up quickly.
Some have great luck using different available tachometer filters to clean up the signal sent to the tachometer. We have not experimented with these so we have no recommendations in that regard.
Best HEI for compatibility we've found is the Actual GM HEI with GM internal parts all being OEM.
Paul Jr @ GMCPauls