With a 4.11 and a decent tire, ie: 275/60/15, it will likely get better mileage then it currently gets, it becomes a matter of efficiency, with a 3.42 gear and a .70 OD its likely lugging along on the highway unless your running a small tire or cruising at 70+MPH. Anytime you can keep the motor in its powerband it will be more efficient.
As for the 93, once you get alum heads on it that will be a non issue, the motor will want 14-16 degree's initial timing, 36* total, a top notch cooling system, ie: proper rad, fan-shroud combo and this thing will run on 87 all day.
And that's how you get your cake (fuel mileage on reg gas) and eat it too (low 13's)