318 safe max RPM?

I think the best thing to do is talk to a trusted engine builder. There's a method to get higher RPM's that has to be followed. Plus you'll waste a lot of them depending where your power band is. There's a reason that 5500-6000 is so very common because above that is going to be wasted unless you build it from the ground up to have the power band up in the higher RPM range. Read this page to see a 6500 RPM + Bulid in action. Oh and it peaked at 406 HP @ 6300 RPM
http://www.popularhotrodding.com/enginemasters/articles/mopar/0667em_mopar_318_engine/index.html