if i buy a rage header or a hytech header for my stock gsr, would i be able to use any cat-back or a special fabricated one?

 

Well , it depends on the exit diameter of the header. Once your know the diameter then you can decide on what exhaust you would like to get.

 

the flange is what matters. the diameter of the header and exhaust only restrict the flow of gas, they don't determine fit. make sure that the header has the same flange as the stock header (and has the correct dimensions) and it should fit the stock exhaust.