When I bought Steffs Challenger and was running it NA, we had the same issues you are having. I had several conversations with some in the know guys and the general consensus was that the stock cam core was inducing a ton of valve train instability. With the weight of the pushrods 9/16 and the valve and the spring pressure needed with 7500=8000 RPM the cam was twisting I did verify after 20 runs 8200 rpm trap the cam was a pretzel. The single biggest upgrade according to Bishoff and Alan Johnson was to step up to a 55mm cam core. They both told me the B1 intake was not the thing holding the combo back. I never got a chance to put the 55mm cam in I bought it and just gave up on the program. Thought I would pass this info along.