But how reliable you looking for it to be?
I'm Sure i can handle that but i personally wouldn't put it that high unless i was planning to rebuild it all the time
Most of the big power stock block b16/18 I have seen seem to break around 450whp+ and most of the time its the stock sleeves that let go, its all in the tune.
Mid 300s are definitely achievable, look at what some of the CRX boys have been doing for years. But as said if you want above that, you'll have to build a block so why not change them for better items anyway!