Don't spend the money on getting a stock manifold. You have 2 good options:
1) Buy a header for LS/ITR and mate it to your GSR cat and ITR catback. GSR cat and LS/ITR cat are the same length but the GSR cat has a hole for the primary O2 sensor which you will need to plug since the LS/ITR header has the O2 hole in the header.
2) Just extend the ITR midpipe by 4.5-5.5" to mate onto your stock cat and manifold.

This info is only applicable for USDM length stuff... JDM stuff is a different animal.