A tachometer circuit is fairly simple in that it converts frequency to current to drive the meter. It is made up of a handful of resistors, capacitors, diodes, a bridge rectifier and perhaps a transistor (or two). In all likelihood, this tach was designed to operate with points and condenser and will not work without a filter to address the high frequency interference generated by the HEI.

This part should fix the problem (assuming the tach has not been damaged):

New Vintage USA 99004-04 - New Vintage USA Tachometer Signal Filters - Overview - SummitRacing.com

If your attached to this tach, it may be worth a try if you want to part with $33 plus the handling charge.

Good Luck,
Glenn