Daily Driver Engine 318 vs 360, what one?
Wait, wait wait... are we seriously talking about comparing a couple of MPG difference when it's going to cost big bucks build a 318 that's reliable at that power level?
Also, the 25lb difference is utterly neglegible.
Build a 360, 4spd if possible (or pick up a trans with overdrive) and some "highway" gears. Easier/cheaper to hit the power levels. Neither will provide decent MPG is that 425 horsepower is exploited in any way.