47-55.1 '50 3600 re-do. Shortening things up!

[dsraven]

if you have the equipment or access to the equipment you could disconnect the fuel system so the injector rails are isolated, then apply system pressure to the rails with the tooling and see if it holds pressure. you gotta have the fittings and a pressure source though. i suppose compressed air would work. same kinda idea as testing a natural gas line to a furnace etc. you gotta know the system pressure but that should be easy enough to find online.

[dsraven]

not familiar with the fitech system. does it have a schrader style test port on the injector rail? if so you could do a quick test by installing a gauge there, then turn on the key so the system primes and builds pressure, then key off and check the gauge to see if the pressure drops. this isn't the best way to check because there are other ways the pressure could drop, but it could give an idea. next up would be to check the area for any obvious fuel leaks around the injectors but, this only really shows external leaks. next up would be to pull spark plugs and look for a wet or sooty plug. next up would be to pull the fuel rails and injectors, leave the injectors connected to the rails. dry the injectors and watch the injectors for leaks while a friend turns the key to on, not start, to prime the system with pressure (again, not sure how the fitech injectors are retained to the rails so this may not work if there are no retainer clips, just the rails being bolted down is enough to hold the injectors in place).
otherwise pull the injectors and have them tested commercially, or replace them one at a time until you think you have the culprit, or replace them as a set.
another thing I have done is to listen to the injectors, one at a time, with a stethoscope or a long screwdriver placed against your ear and the injector, and see if they all sound the same as the engine is running. pull an injector wire connector off one injector at a time and check the engine sound, then plug it back in and try another.