I would replace one piece at a time if you can. If your front clip is straight and square then you should be able to remove one side and install, for example, a whole frame rail and then get the measurements back to what they were before and tac weld it in place. Double check everything prior to finish welding. I have not a lot of experience doing this kind of work but I have been doing research for months and this is the route I would take. I have been welding for years so I can trust that. All of the above suggestions should be used. Next week I will start ordering new metal for the front of the car and work my way towards the back.