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What's important is the number of teeth and the rotation. IIRC, the gears where the speedometer cable exits on the left side of the transmission are typically left hand rotation, and opposite for the passenger side. I had gotten that info a catalog. Not LMC, but another one and the name escapes me at the moment.
Not a stock tire size. Your truck should have come with an 18 tooth driven gear and a 7 tooth drive gear in the transmission. With the 255/70R15 tires and 3.73 gears you should use a 19 tooth gear.