Unless your subrails are extremely bad, I would try to repair the originals. If you use repros, I don't think there are any holes for the body crossmembers, B pillar parts and other holes needed to properly align everything. Phaetons are hard enough to get properly aligned so the four doors fit OK. I have a 29 phaeton that had the subrails and body crossmembers in real bad condtion. I welded them up and dressed them down. It took me a long time to do it, but the holes are still there to make sure the doors, quarterpanels, etc have a decent chance to be placed correctly. Just my opinion.
Rusty Nelson