Upgraded to HEI and now the tach jumps around

I am not sure what exactly the same means. Same as before HEI, or always waver of 200 RPM.

Tach basically counts ignition pulses, more pulses per time, greater the reading. A waver means noise is making more pulses, but only for some cylinders. That suggests some plug wires could be too close. At some point in higher RPM, the additional false triggering, will waste time in coil charge resulting in weak spark.