The only difference is the king pin size.
True, but the axle is a more heavy axle and was used through 54.5 pickups. It is a good upgrade for a 37-41 pickup.
The changout Pre 68 Dave writes is the way to go. The 51-54 car backing plates bolt to the 51-54 truck axle, thus making it easy to use the 51-54 car backing plates with the brakes and you get the Bendix system, a better brake system then the one on your truck. The spindles on a 51-54 truck axle will accept the 51-54 car backing plates and brake drums.
It's a pretty easy change over - no welding or any "heavy" type of work. It's a Saturday afternoon job.
