360 build

i already have a cam its a 272 and i was thinking more of 3.23 and 300 to 315 im happy with i read the thread that i can get the gas mileage in the 17 to 20 range which is nice out of a 70s car if it actually does that i live about 20 miles from anything so mpgs is kinda needed and i was told that 3 inch exhaust is over kill even though i have a 3 inch cherry bomb extreme out a 3inch exit on my ford 4x4 with a stock 302 and it sounds like a muscle car with just one pipe