Fuel mileage will be crap either way, and since your truck has the 440 in it already, I would stick with it. I'm guessing your 440 is a garden variety low CR 70's smogger 440. Since you don't know what that cam in there is, I would change it to a 10230700LK Lunati cam. It's a mild cam but good for a truck/puller. Stock intake manifold or an edelbrock performer (not the rpm) if you must have aluminum. Headers if you want, however in this case I don't really see them gaining you much of anything. A 2-1/4 dual exhaust would be ample and give you a little sound. Your key will be in the distributor and carb. Recurve your distributor properly, I agree with the numbers dezduster gave. The holley 700dp is a good carb, just not a good carb for a truck or a tow rig. Holley's aren't known for fuel mileage and a mechanical secondary carb is a bad idea on a tow rig. I've had good luck with my thermoquad, others have had good luck with their quadrajets. There's also that holley street demon that looks very nice. I wouldn't look at a conventional holley, even a vacuum secondary carb, for mpg in a tow vehicle.