You indicated your 6 cylinder truck has a 3 speed transmission. If it works OK you will not gain anything by purchasing one from a V-8 truck as they are the same transmission, same ratios, same mounting. If it is priced right and you feel you need a backup, go for it. As Bill stated, the factory overdrive is a good choice for all around driving and considerably better for highway speeds. There is no reason to change the rear end with this set up.
Fred
