The 1954-1962 235 and 261 blocks are all the same length, but the overall engine length of the 55-62 engines is longer that the overall engine length of a 54/55 235/261.
OldSub is correctly describing differences in total engine length and I think that Greg is thinking block length (which is the same from 54-62 for all 235/261 blocks).
The 1954/55 235/261 engines have a shorter shaft on the water pump. The 55-62 235/261s can be easily adapted to attach the earlier short-shaft water pump on a 47-55 truck, Or, a special short-shaft water pump of later design may be obtained from a few vendors (that pump will work but the fan is not in the original location).
The front motor mount plates might be different over the years but this is easily swapped or converted.
If you bolt the right 216/235/261 clutch housing on any 1954-1962 235/261 block, it will mount in any 1947-1955 Chevy truck.
So, the overall length of the 55-62 235/261 engines are longer than the 54/55 engines, due to the water pump.
As stated above by OldSub, the 54/55 235/261 is the easiest swap into 1954 truck; adapting a 55-62 235/261 requires relatively easy adaptations; and, swapping to a GMC engine requires more significant adaptations (to fit into a 47-55 Chevy truck).
Tim
