GM put tach filters on all of their tach-equipped HEI models back in the day, and there was a reason why They are an RC filter that is designed to remove high frequency noise from the tach line. They smooth out the spikes in ignition system. I would try that first.
Here is an ebay ad for a factory GM filter:
http://www.ebay.com/itm/1978-TO-87-T...df7b56&vxp=mtr