Aerodynamic upgrades?

A slightly nose-down stance, about 5* I believe, tends to help aerodynamics and a front spoiler will help minimize airflow getting caught up in the "rough" underside. Similarly, tractor trailers have air dams under the box shaped in a "V" to direct airflow around the trailer's rear tires.

There was some controversy about spoilers on late '60's Camaros in Stock & Super Stock drag racing a while back. Some racers were using the front spoiler without the ducktail rear spoiler that was part of an option package. The reason was that while the front spoiler helped aerodynamics, the rear spoiler actually created drag! I believe NHRA stated that either both spoilers are used or neither.

Readjust fenders & doors to minimize the gaps.

Synthetic oils - a little thinner, if possible, in the engine, trans & rear.

Cost vs return does need to be considered. Most benefits will only be obvious at highway speeds.