3.23 or 3.55 Behind a 318

What is a “Commando box?”

Anyone have experience with 2.76s or 2.94s behind a commando box? I'm looking for something a little more fuel sipping, but I don't wanna be burning the clutch when I'm just around town or in traffic. I do still wanna have plenty of fun with it. So if not that, then I may be going with an a833od+ 3.55s
(Car is 2700lbs the 360 makes about 380ish hp)