As 69Cuda correctly posted above, the 261 was an optional engine in 6xxx series 1954 trucks.

The engine serial number pressed into the block behind the distributor will ID the engine and size of truck (if it is a 1954 engine).

A 2-speed rear end was an option on all 6000 series trucks.

There were no GM RPO (Regular Production Option) auxiliary transmission in 1954.

http://chevy.oldcarmanualproject.com/chevyresto/54115.htm