what combo = decent mileage+ power/torque

ok what do you guys think? 318,349,360,408+ cid? what heads? lets keep under $2500 bucks or so? something that will idle well enough to run power brakes and doesnt need a lot of converter and let say a gear range of 2.94 to 3.55 .......i have thought about the 349 stroker but then whats the point when you can build a 360 + for the same money ......but then i dont want to get 10mpg on the interstate either....