If mileage is the primary concern, a magnum 360 will make max torque 1000 rpm lower than a 5.7 hemi. Build the 360 with some Kb107 pistons and magnum heads set up for .040 quench and mileage and torque will both go up from a regular factory motor. I don't think the hemi is even an option for most people with only a 3-4 k budget.
