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All model year 2001 and later C5s (coupes, verts and Z06s) received the same newly designed composite intake manifold so your 2004 already has it installed from the factory.
As you didn't specify which LS6 intake, I assumed you were asking about the LS6 intake manifold. Then, I got to thinking that you may be asking about the LS6 air intake. If this is what you are asking about, some of the folks here that have actually installed the LS6 air intake on their coupe/vert may be in a better position to talk to you about this and their results. Here's a current thread going around C5 General on the LS6 air intake: