I have an aftermarket Tic Toc Tach on my car with HEI. No problems. The way I remember it, you have to order the correct wiring harness so it plugs into the HEI "port".
Here's the note from the Factory Fit website who I got my wiring harness from:
All stock harnesses can be modified to incorporate HEI or alternator modifications even if not listed. Please place a comment in the "Notes" section of our webstore order form stating which modification you need for your vehicle.
Tach works fine...
You can use a tach filer if need be. I didnt' need one. Bought my tach from Classic - made by OER.