I was told to use a GSR header for my LS/VTEC build in my 99 Si but whne I received it the downpipe isnt as long as my old b16 exhaust manifold so it isn't going to reach my Cat. Which header should I be looking for?

 

An LS or ITR manifold will be the same length as your B16 manifold was, however the angle of the flange at the cat is different, so you'll need to modify your existing cat or use an LS or ITR cat.