Hello Brian,
I have gone through old posts to see what I missed.......
Actually it converted the engine into a partial ramjet with capability above Mach 3”, the J-58 taken in isolation, as on the test bed, does not have a ramjet mode, partial or otherwise
The engine didn't need any ram to run in its cruise operating mode, just the right inlet temperature and pressure. It did it on the ground. Nobody would have signed it off as flightworthy if it hadn't.
Whatever the operating mode of the engine as part of the complete propulsion system at cruise it would also have had to have been run in the same modes in isolation on the ground to qualify it for flight since it was the primary thrust producer for the aircraft (not test engine on an FTB).
Since it ran at cruise design-point inlet conditions on the ground it would have operated in the same modes as in flight. See use of J57/J79 exhaust, etc to condition inlet air
SR-71 J-58 Powerplant
I presume it would have had to run a 50 hour endurance test (called a PFRT) to qualify it for flight throughout the envelope including some hours of continuous running at the cruise design point plus some margins.
The engine would have been stripped and inspected before every detail in that particular design build got the OK to fly.
The value of the cruise part of the PFRT is that the engine is running in cruise mode (and will show up problems) but it happens to be on the ground with no ram from a Mach 3 intake.