Before you worry about changing things I think you should get an accurate base line. Either your speedometer, your tach or both are incorrect.
Beg, borrow or steal a GPS unit. Set it to display MPH and take it out on a highway without a lot of traffic. Run the car at 60 MPH as displayed on the GPS and record what the speedometer says. Do the same at 70 as indicated on the GPS and a few other points, say 40 and 50.
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My Pontiac is a '57 GMC with its original 347" Pontiac V8 and dual-range Hydra-Matic.
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