Depending on the year, 235's were max horsepower rated at anywhere from 3700 to 4000 RPM. A good condition 261 (same stroke as a 235) should cruise all day safely at 3000-3200 as long as it stays cool and has good oil pressure. These engines in medium-duty trucks would run all day at a governed RPM very near the HP peak. Most people are too timid to run 'em that way these days!
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!