I love the trans in my '49 6400. Truck is all original so I assume it has the sm420. Very quiet transmission in all gears except granny low. You can't rush the shifts, but I rarely "grind 'em". I hear the sm420 is a popular upgrade in older Jeeps. My buddy bought some kind of conversion kit for his old Jeep a few weeks ago. From what he saying, those old four speeds aren't nearly as easy to find as they used to be.
