Just a note: 4 main bearings obviously isn't the best design, but it's not an inherent RPM limiting factor.
The "slant 6" 225 has an even longer stroke than the 235/261 (4.125" vs. 3.9375") and does 5,500 RPM quite nicely.
An industry guideline for highest sustained cruising speed (assuming water & oil temperature are OK) is 2,500 feet per minute average piston speed (15,000 ÷ stroke = RPM). For a 235/261 this is 3,810 RPM.
