Looking at the factory maximum HP ratings of several mid-1950's 235's, they're specified at 3700-4200 RPM for various year models and transmission/rear end combinations. Since the 261 has the same stroke as a 235, similar figures would probably apply. I don't have 261 factory specifications handy at the moment. A 261's big advantage over a 235 is increased torque, not a lot more top end horsepower. Above those speeds, HP begins to drop off pretty dramatically as the "curve" goes above maximum. Torque tops out about 2000 RPM for most applications, so the "safe" operating range for best performance seems to be in the 2K-4K speed range. 4500-4800 won't hurt it- - - -but you're just spitting into the wind, power-wise.
While it's possible to make a stovebolt engine run above 4K RPM, it seems to be pretty pointless unless modifications such as camshaft changes and cylinder head and valve upgrades are done to move the HP curve to a higher RPM range. Then you end up with an engine that loses massive amounts of torque and pulling power in pursuit of a rather mediocre HP gain.
Jerry
