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On a points distributor Pre-1975 it hooks to the coil, if you are using a points, I would ditch it.On an HEI distributor it hooks into the slot marked TACH on the distributor cap.For the gauge overlay, it looks like what bigblock73 is using here:http://73-87.com/7387garage/interior/gauges_jeff/gauges01.jpgI think tvBlazer78 is using a polished version of the same.