I'd recommend a 292 engine and a NV 3500 5 speed overdrive transmission, coupled up to a Dana Type 60 3.73 or 4.11 rear axle. You'd still have the inline 6 engine, but with enough torque to handle the towing chores nicely and an overdrive gear for good highway cruise speed. If you're considering V8 power, it seems you're not a purist for a period-correct running gear. The 292 was designed for load-pulling and/or trailer towing, and it will cruise well when paired up with the right geartrain.
Jerry

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