For the initial shakedown I plan to run those factory L76 headers with a dump pipe and O2 sensors just to get it running. We're not talking about driving anywhere, just a bit of up and down the driveway.
As mentioned above, speak to Warwick Harrison about header plates. He told me before xmas that he was close to offering an exhaust setup for his customers running the new GM engines, to be custom built by someone up that way.