I would not go any bigger than a RVgrind cam, unless you plan on going back to old school emissions, as far as gearing you need to decide on what you want most, good pulling power but bad mileage or good mileage and low pulling power, the best overall ratios seem to be around 3.73 in most GM trucks, but the choice is yours. Remember more power to the wheels equals less fuel economy and quicker redline on the engine. HTH
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