what combo = decent mileage+ power/torque

Budget will be your limiting factor. Parts alone will run you up to $2500. Do you plan on doing all the manual work yourself? Any type of stroker would probably overrun your budget. Cost to build 349 vs a 360, may be cheaper to go with a 360 with reuseable internals.

Otherwise a used magnum engine with some bolt-ons (cam, headers) might get you there.

2.94-3.55 is a wide range. 3.21 or 3.23 may be good performance/economy balance.