[aerohead]

At the time I moved away from California ( 1975 ), it was still legal to drive 45-mph on any freeway. Is this going to be your target velocity?
You know where the engine's BSFC sweet spot is. If all efficiency gain is to come at 45-mph, and only from aero drag reduction, then, after streamlining, the Tahoe's gearing must be such that is absolutely matches the load you're seeing now, keeping it on the same BSFC pounds per brake horsepower-hour island, perhaps requiring a Gear-Vendors overdrive unit if you can't find a ring and pinion tall enough.