The cathedral port heads on our LQ9's are the limiting factor of horsepower. You will not "run out of manifold" when using a LS6 intake on the original heads. If you ever decide to use the LS3 or L92-style heads, you'll have to go with an LS3 style intake. Get the LS6. Yes, it's plastic, but if GM used it on the Corvette, I think it'll do fine. Used, they're only 300 or lower on eBay all the time, even with low mileage. They make great power, and torque. Honestly, the Trailblazer SS manifold did even better than the LS6, but it's FUGLY!