Nothing - keeping stock intake manifold.
When I still had my 2000 LS1 Z28 (ONLY the '01s and '02s came with the LS6 intake from the factory), I was looking for an LS6 intake manifold, but people wanted crazy coin for their used ones (and one never knew if it was damaged from using 'wet shot' spray setups until it was received).
Plus, the new ones were more than half the co$t of the smaller, LS1/6 application FAST deals.