The 1955-2nd (255) will have block/water-pump fitting like the 1956 and later.
The 1955-1st will have a block/water-pump fitting like the 1954.
They are both truck blocks, so they have both have a metal cam gear and mechanical lifters.
All 1955 (and later) cams and distributor-gears were the same material (most, if not all 1954's were the same material as in later years). However, the 1956 camshaft style changed to a higher lift design (the 261 camshaft?) and the higher-compression "848" head was introduced (resulting in greater horsepower).
As described by Tiny_Jaime, the change to non-adjustable main bearings began in 1956.
In case you are interested: the F255MB originally came in a 1955 3800 (1 ton) Task Force truck equipped with a Hydra-Matic transmission; and, the F55X originally came in a 1955 3100 (1/2 ton) Advance-Design truck with a standard transmission.
