If you keep it under 6k the stock block is surprisingly strong, rpm and abrupt shocks kill them. You might want to check out
http://www.panhandleperformance.com, their girdle fits great with a canton(POS) pan.
With a totally stock bottom end I'd say 450fwhp would be about the limit. If you have it blueprinted, balance, girdled, and get a good balancer you can push 500-550fwhp safely.
Good luck
Brad