Looking for a Allison 250 fuel nozzle test rig or the specs for one so it can be built
Are you referring to a test rig for nozzle overhaul and flow calibrations? If so, it's not really an item one specs out and builds. But here is the RR P/N for a factory version 6893374. A number of shops use 3rd party bought equipment that handles a variety of fuel nozzle types.