I have a 70 challenger w/ac and a 71 dash with rallye cluster and no ac. It would be cool to have the tach in the dash, so whats different besides tach wiring harness? How different is the wiring harness? Any other issues I might have like defrost ducts? What about the dash frame? The car hasn't ever had air since i've owned it, so that wont be an issue, plus I like the look of the dash pad without vent holes. Thanks