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send yr t56 to someone and get a differnet rear extension fitted that has speedo sensor provision even on the ls1 t56 we sometimes get a custom rear housing from usa that can run both electric speedo sensor for ecu and cable drive for older cars like mercs and stuff for when its too hard to put electric speedo in
might cost abit but much less work than stuffing round with reluctor rings and sensors
on the axles or tailshaft