Most likely, it'll be a 3.90, the most common gear in a '56. There was a 3.08 used in cars, starting in '58, but that 3rd member will not swap into the truck housing. There is a considerable differnce in not only the size of the housing and gear, but the shape is different, too (round for truck, flattened oval for car).
I would strongly suggest an overdirve trans with the deeper gear, be it a factory od, T-5, or a 700r4. That would give the best of both worlds, low rear gearing for in town driving, then a higher final gear for highway driving.
