Not sure about the shifting up and down, but I think those low rpms might be lugging the engine somewhat. He will be running not only the overdrive ratio, but pretty tall tires and a less than aerodynamic body style. It just might overwork the engine pushing a 70 mph wind at those rpms. An engine should come into the range where it is running most efficiently, where the cam is starting to work . If he puts any kind of cam in that combo at all he will be operating below it's intended range.

None of the above is scientific, just my gut feelings.

Don