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It's interesting to see this dyno graph. First, it seems that the dyno operator does not know how to dyno a car period as there are two shifts that occurred during these dyno session. Secondly, the upper rpms indicated that you were either spraying a small shot of nitrous or something because you nearly gained 40rwhp in a small range of 400 rpm.
Good point. I will try to confirm for you sometime this week.
S&S used to make header for the GS4. If you look in the performance section you will see the long thread on it.
S & S is dead....a new vendor from here picked up there jigs with promise to have a modified new design on the old S & S header soon. try the performance section V8Rdnlow