digital meter

All the tach wire actually is on a "real" (GM) hei is a connection to the coil NEG terminal. I have no answer unless it was RF interference. Try twisting the test leads, position the meter away from the HEI, lay the leads down along the engine, and clip them to ground and the tach connection with as little "untwist" that you can manage.

Can/ have you tried it on another vehicle?