I would leave the XR3i gearbox in as I'm pretty sure Ford used the same bolt patterns, you might have to double check that the clutch and flywheel fit though. The reason I say do it that way is that Im sure that RS2000 has different drive shafts and Im not sure if you can put them into the mk4 hubs, Might be able to though, Im sure someone on here will be able to help.