JB,
They're PC'd from the factory, so I don't see what the problem is. Asside from that, the Stainless marine manifolds I used to run never got that hot externally. You could always put your hand on em for a couple of seconds, so I doubt they were anywhere near 300 degrees.
On another note, I forgot to give you props for your motor build up on the other thread. Very nice HP and TQ numbers for that not so small block
Good luck,
Chris