Bruce,
The 55.1 was a ony year only rear, as far as the springs and perches go. The same rear was used till about 57 in gmc, and 62 for chevy with different spring widths. This rear is known as the Eaton HO33. The possible ratios are 4.11, 3.90, and 3.38. These center sections will interchange with the earlier 1947-1954 closed drive axle housings, and may be backwards compatible to 1940. 3.90 will be the most common gears for the open HO33. 3.38 gears are available through some of the parts suppliers, but are often on backorder.
As for brake configuration, I believe 1953 is when the Hucks were replaced with Bendix brakes, but I may be off on the year. All 1955 1/2 ton rears would have had bendix brakes.
