One of the reasons that the "bell" on the end of the SM420s is hard to find is that they tended to break from the torque reaction of compound low or reverse. On the 3/4 tons I've seen, the drive line is a hybrid: enclosed from transmission to carrier bearing mounted to a cross member and open from there back. These didn't break the end housing.
In my younger days, these pickups were still in common use and the country half tons were often used far beyond their intended limits. Most of the town or city half tons were probably three speeds and the country half tons were mostly four speeds. A friend of mine back in the day used his 51 half-ton for hauling water when the well went dry - a 325 gal oil tank, hopefully never used for heating oil. He did that till the frame cracked. After that, we used my 1-ton with the same tank. I was sharing a farm house with him.
He got the frame welded up and that winter, hauling firewood out of the woods with chains on, he cracked the bell on the back of the transmission.
