I'd measure the distance from the rear crossmember,to the hole in the frame rail,where the shackle mount, on both Road Runners,and see if there the same distance,that should tell you if the replacement rails are correct,compared to the car that the springs fit in correctly.

70 Charger R/T SE 472 Hemi 70 Charger R/T convertible 70 Charger R/T V Code Sixpack 69 Charger R/T SE Sunroofcar 68 Charger 383 68 Charger 318 71 Charger R/T 70 Challenger convertible 71 Challenger convertible 71 Cuda 340 09 Challenger R/T Classic