the ls6 manifold will help if you have a 5.3-6.0 and have a cam.
in a 4.8 all I would do is port your stock tb, you wont need anything bigger and the little bit of extra low end tq means allot for that little motor.

I just picked up an ls6 intake with a ported ls1 tb and will be installing it on my motor. but I have a head/cam/full bolt-on 6.0. on a 4.8 or 5.3 I would just stick to the stock one and do the small free upgrades too it.(coolant blockoff and port the tb)