The stock internals are as bulletproof as the GTE motors (I hope I didn't just jinx myself). It's the headgaskets that are the problems. They're not made for boost, so they will only hold 10psi or less.
I'm going to run 10psi for right now, which should get me to around 300whp. It will be a very fun DD for me. I've got some stuff to do to the head (you can see the FFIM up a few posts, would like to upgrade fuel, headgasket, ARP studs, etc.), and after that, I will be looking for mid-upper 400s out of it.
I may end up brushing the 500whp mark with it eventually, I don't know yet. I'm not really worried about that right now, I just want to get it back together so I can drive and enjoy it.