At what HP /rpm level are billet main caps and or stud girdle needed?

I wouldn't even consider NOT runnin that stuff with the engine you've outlined. I'd consider it part of the assembly right off the bat. But to answer your question, I'd say a solid number is an honest 650HP, especially one that's gonna get beat on. I would also include a partial hard block fill to the bottom of the freeze plugs. Lastly, don't skimp on the cooling system.