I am wondering what the internal limits of our 24valve's before we start blowing head gaskets, needing head studs, etc...

Is it somewhat reliable to get to 450 hp with out having to do internal mods?

 

Yes.

You should be able to do 450 hp at around 45psi. (depending on how you go about things) Studs aren't really needed untill 50psi. If I was gonna be topping 40 PSI a lot, I would want them just for peace of mind though.

 

If your going to be doing 40+ psi alot then studs are definately worth the $450. The internals on the cummins are incredibly stout. O-rings and studs in the head and you can reliably get 600-650hp out of your truck.