I still like the lowered profile tire, alloy rim approach.

Dave is dead on about EFI. Electronic fuel injection and ignition is self compensating instantaneously to correct the combustion process. Carbs and dizzies can err a lot, and are at best a rough approximation of ideal control.

The formula Vee approach to squeezing out more power is another good concept. By various means the energy required just to turn the mechanical mass of the engine can be reduced. Roller rockers and lifters, timing gears vice chains with an oil groove cut in the gear teeth to relieve pressures between the spinning gears, lightened flywheels, special additives to reduce friction, and numerous other tricks are out there.

All the way out to the wheel hubs - some bearings are better than others. Geo Metro's for example have sealed machine bearing cartridges instead of adjustable wheel bearings.

Standard coolant pumps can be replaced with electrically driven ones - but that is more of a strictly racing measure.

Definitely an electric fan with a thermostatic temperature control. A fan does not even need power above a certain speed. In fact - it's a pity electric fans are not made to serve as secondary alternators at highway speeds.