This is a topic that can be debated for days. It mainly comes down to the particular block. Some blocks have bad core shift, weak webbing, etc. be it two or four bolt, others can take tons of h.p. without ever having any problems. Personally I would not feel safe over 450 with a two bolt block.
The Chevy factory testing sessions of the first 15 years of the small block concluded that 300hp.+engines must be re-enforced with 4-bolt mains and high-precision machined and balanced cranks and attendant rotating assembly's to live up to a 100k miles without a major internal malfunction.