I'm looking to upgrade the headers on my WHll Statesman. I currently have a twin 2.5 inch system from the cats. The engine retains the stock headers/cats. I'm working to a tight budget so I'm chasing a set of HSV headers. I know that the VYll ones are the better choice however I want to know if these will bolt up to the stock cat pipes or whether they need modification? Anyone done this mod? Thanks!