Great car for a highway drive. My DX (no lean burn, worse aero, higher rpm) will do low 40's with the (aftermarket) cruise set at 65. Yours should do even better.
With a highway drive like that, weight reduction will gain you very little. It mostly matters at the start and end of a trip. I'd put the spare tire back in for peace of mine.
With a highway drive, aerodynamics will be important. Those HX rims are nice and light, but they're not air-friendly. Some kind of smooth cover would help. At one point I had cutouts from old election signs zip-tied to my rims. I now have semi-smooth hubcaps, but I drive slow routes so aero matters less to me.
Things like deleting one exterior mirror, or a smooth belly panel, or "skirts" over the rear wheels, or others in the "mods" list above will help on a highway drive.
In KY, you should have some good downhill stretches for coasting. At idle, the engine is turning ~800 rpm, vs 2500 rpm in gear. You have to burn extra gas just to keep the engine turning so fast.
You can program a gauge into the SG to tell you when you have lean burn activated. Try to keep that ON for most of the trip.
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