Most likely, you have the 3.90. That was the most common ratio found in them, regardless of engine. If you go automatic, you do not need to go to a floor shifter, the column shifter can be adapted. Alternatively, the SM319 is an excellent option. It's basically the same transmission as the base 3 speed, except that it has a Borg-Warner R-10 overdrive unit in place of the tailshaft. It's a direct bolt in if it came from a truck, and one from a '55-'57 Passenger will also directly fit with a couple of alterations, mainly the shift cover and arms may require swapping from the original transmission, and the lower mount bolts go in opposite from Truck, so you have to use a bolt and nut setup. Works great with the 3.90 gear.
