Aerodynamic upgrades?

I would not change anything. Driving technique is a major factor. Very slow throttle changes to prevent added enrichment over what's called for at that point in the map. Parasitic loss is lowest at 1:1, so running a numerically lower axle ratio to work with 4th gear at cruise speed is more efficient. Numerically lower ring and pinion usually have thicker teeth so a lighter duty axle can be used for the same strength. Removing the side mirror(s) and wipers, flush fuel filler and trim, reduce the grill opening, add an air dam under the bumper to reduce the amount of air flow under the car and matte black skirts under the rockers (to hide) to do the same, drums instead of discs for less rolling drag, lengthen the rear fenders and trunk lid with flat extensions (a longer shape is generally more aero), higher tire pressure, narrower wheels to allow rear fender skirts, tuck the rear of the front fenders to get the high pressure out of the wheel well, etc.