whats the best engine for both performance and mileage??

FWIW my Duster with the engine in my sig gets 18 MPG on the highway with a TCI (not the best, lots of slippage) 2500-stall converter and 2.76 gears, and that's in an area with lots of hills and my lead foot lol. With a 5-speed manual and 3.23 gears I'm sure it could easily get over 22 MPG. Building a 318 instead of 360 and roller cam instead of small hyd. F.T. would improve it even more along with carefully ported aftermarket heads and a TQ or new street avenger carb.