One of the problems encountered with the Unilite distributors is that they are supposed to have a ballast resistor installed in the wiring in series with the power wire. The distributor is not designed to run without the ballast resistor. See ERA's website:
http://www.erareplicas.com/427man/wiring/msd.htm
Without the ballast resistor, the unit will run for a while and then the module will burn up. If your engine is still running, that's probably not the problem.
More likely, depending on the specific models of ignition box, distributor, and tachometer, you may need a tachometer adaptor:
http://www.msdignition.com/1troub1.htm#anchor335804
Hope this helps.