You say you want to be able to haul a load and cruise confortably at highway speeds so some type of overdrive is a must. The 3.90 rear mentioned is a good choice as this gives a 2.73 final in od which is about as shallow a gear as can be pulled comfortably with a load and a stock 350. Radial tires at recommended pressure, synthetic oils and fluids, free flowing exhaust, coolant temp as high as possible without losing any, FUEL INJECTION, no sun visor/west coast mirrors/bug shields or other air catchers and your driving style will all be factors. Why the hang up on 350's when the newer LS engines are so much better and give great mileage right out of the box. It's the reason GM canned the old sbc design. Price and electronics would be my guess but the results are undeniable.
