Best possible mileage?

My 66 Dart, 273-4 get's 18 hwy. if I drive it nice. My 70 318 Swinger get's 18 hwy. pretty consistently. It has dual exhaust and the stock 2 barrel. I am curious how much mileage may change and in what direction with a LD4B and a 650 AVS2. Both are 3.23 SG cars but I run taller 15" tires.