Worked example
300 lb-ft at 5,000 RPM is about 285.6 hp. The same 300 lb-ft at 2,500 RPM is about half that power.
Why torque and horsepower cross near 5,252 RPM
When both are plotted using lb-ft and mechanical horsepower, the unit definitions make the numerical values equal at approximately 5,252 RPM. It is a unit relationship, not a special engine behaviour.
Peak torque is not enough to calculate peak power
You need the torque at the RPM of interest. Using a peak torque value at a different RPM can create a power figure the engine never actually produces.