are you currently having problems getting pressure down?if you have a decent after market fuel pressure regulator you shouldn't have to mess with the factory return line,have used factory return lines on quite a few different combos and always been able to get rail pressure down to 4bar as it should be.Also not sure what line you plan on using to feed the surge,if you are using the factory line make sure you get rid of the factory filter and replace it with a stainless element filter.