My logic wants to build in extra strength in the drivetrain to ensure that it is not only capable of supporting short bursts, but also to be extra reliable in the long haul, free from long-term exposure to high stresses that cause metal fatique.

If the factory engine is 300 HP and the strength of all components is rated at >400 HP before catastrophic failure.
__AND__Major engine/drivetrain stresses capable of causing wear and metal fatigue is just short of catastrophic failure occurs at some level below 400 HP (with factors of time, heat, and engine torque all considered).___AND___I want to build a 500 HP engine that is capable of sustaining occasional bursts of speed with comparable torque,

Then how much should be calculated as an overage to ensure not only avoidance of catastrophic failure, but reduce the stresses due to metal fatigue, prone in lesser designs.