The difference is that in 54 they went to a flat top rail, instead of the "Y" angle that was used from around 1949 - 1953. So the top support brackets are different. Also, the 1954 bed is taller, so you have to trim the vertical mounting brackets. You'll know it is for a 3600 by the lug pattern - the 3100 is like an equilateral triangle, while the 3600 has two long legs and one short leg.
The biggest problem is that it did not come out far enough from the truck. I am using 215/85s and they might be wider than the original? So, I went to a welding shop, and they lengthened the legs on the bracket. No big deal. But I still needed some shim washers to make everything was 100% vertical. If you are lucky enough to find a bracvket at all, you can surely modify it to fit.
Hoped this helps.
Harry
