The Stovebolt Site Forums The Shop Area General Truck Talk Differences between 54 and 55.1 3604's

I have a technical question regarding the differences between 54 and 55.1 Chevrolet trucks, but in 3600 form. I know that the chief mechanical difference between 3100's of these series is the open drive line, and, if I recall the spring perches moved to the top of the axle housing as well. What are the main mechanical differences for 3604's between 54 and 55.1? Are there any?
3604 is a conventional bed single rear wheel pickup.

Thanks!