I believe the 261 was HP rated at 3600 by GM. I really doubt that they would specify a destructive speed range to rate the engine in their specs, particularly when the big rigs had gear ratios and governors that allowed then to run at full throttle all day every day. The problem is not with the engines- - - -it's with the snowflakes with sensitive ears who are driving them these days! As I suggested, try a run without a fan and compare the noises you're hearing to what you normally experience. I think you're going to find that the frightening noises are actually external. I'd also suggest at least temporarily fitting a tach to see what the real RPM is at various speeds.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!