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

I would buy an aftermarket block more for its ability to use a bigger bore, than the strength aspect, though that is a very welcome benefit.
If I had a good 400 block (I do) and if the main caps were missing (they are) and I were building a street/strip n/a motor, not some 1200 hp boosted combo (I am) I think a set of billet caps and a girdle and hardblock is a perfectly fine solution..
I bet I can get a stoker kit and some good heads with the money saved.
(and yeah, mid eight second laps with boost or nitrous, aftermarket block is step one)