Keep the piston and bearing clearances close to minimum, and do a pretty good static balance job, and don't worry about anything under 3,000 RPM. I think most of the inline sixes were horsepower rated at or above 4,000 RPM, so that should give you an idea what the engine is capable of surviving. My Motor manual lists the 1958 model car 235 at 145 HP at 4,200 RPM, with a torque peak of 215 Ft./LB. at 2,400. The 261 should beat those numbers by a noticeable amount, and since the stroke is the same as the 235, it should live happily at 3,500 or less for a long time.
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!