Don't use copper gaskets on the manifolds. Matt will confirm this for his pipe/manifolds. I have already asked him about this.
Just a thin smear of high temp silicon and bolt the manifold tight to the 300 barrels. Wipe off with your finger any silicon that squeezes into the exhaust.
I tried to cut an o-ring groove into my manifolds last week and failed, cut the groove too deep. I need more practice on a lathe. lol In my defense it was a pretty old crappy lathe and the chuck was way too big.
I've had a mate fill my failed groove with weld and dropped the exhaust, manifolds and o-rings into the machinist today.