[I know for a fact that the 360 get really bad mileage. I have had dozens of them.] well go ahead and build the 273 , I have a 360 about 425hp runs very low 12's, single plane 750 race holley, that gets about 12 mpg with a 3.55 gear in a 3450lbs car. something is not in good working order on your 360, have you tried to tune it up?