Actually, for the most savings per bang it would make more sense to get the truck more efficient than the car (since it probably has the lower MPG between the two). But that's just splitting dollars and gallons.
I would split the load evenly and have each driver maximize their vehicles using ecodriving techniques (which are more effective/cheaper/immediate than any mods). Do you have to cruise at 58mph? I guess this is your "sweet spot"? Otherwise I would dial it down a notch or two to 55mph just to reduce the drag on your truck (which probably catches wind like any good parachute
).