[mmcleo11]

2.5" sports exhaust system, with mufflers to suit how loud you like it free up the engine a little and let it breath a bit better.

Fit a set of higher ratio roller rockers to the heads, which will improve power output a little but more importantly, make a bit more low rpm torque.

If you keep the cruise control on for the trip, it should be pretty good on fuel over such a long trip.

Also, as you stock ECU is tuned for 91 fuel from the factory, there is no major benefit in running 98/95 until you have the ECU reprogrammed to suit. The only benefit you are getting is slightly better exhaust emissions and maybe a 1~3% improvement in fuel economy. With the price difference of 91 to 95/98, ask yourself if it is really worth it...