I agree with Azrael. But if your not using CCV, create you reflect line, extrude or sweep your runout surfaces, then trim to the original part surf to your runout surfaces twice (1 for core, 1 for cav.) Personally, I keep part data datum, join them all then do my pl, when i need to update the data, i just swap out the surfaces of join i need and everything updates. i find this the quickest and most efficient.