Performance mixed with fuel economy!?

Your title made me chuckle. Nothing wrong with a 318 but if you build it for performance (depending how far you go) you won't get optimum mileage. A Teen with a 4 barrel, proper distributor advance curve, and dual exhaust can be pretty snappy and still get you upper teens for mileage on the highway. 3.55 is a push for mileage unless you have a overdrive transmission. That's a whole other can of worms as far as modifications unless you are running a 4 speed. I have a 273-4 with a 3.23 gear and a 4 speed in my 66 and a 318-2 barrel automatic with a 3.23 gear in my 70 Swinger. Both get upper teens for mileage and run well. No tire burners as far as performance. It is nearly impossible to have it both ways.