Because headers open up the flow right off the heads meanwhile the DP opens up after the Manifolds. And yes to run 3.4 definitely want headers unless you like to take KR up the wazoo.
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04 GTO M6 | HE SS Clutch | Kooks LT's | SLP LM2 | K&N
Soon: APS TT kit
SOLD \/
01 GT | STr5 cam | PT-67 | 18-22psi