Some reproductions are fairly accurate where others require quite a bit of rework to get gaps and body lines correct and still others are so far off that you may as well find something from a junkyard in AZ. For many of the reproductions I’ve run across I had the owner send them back and either made the part myself or repaired the original. At the end of the day you need to be comfortable with the parts you’re buying or comfortable with your skills to get you to your end goal.
With the modifications you have planned, if much of the defect areas are being cut out, then it plays in your favor in keeping the original stuff that fit. My suggestion with respect to the reproduction parts is to check some forums specific to the vehicle to see if there is feedback on which parts may or may not be accurate.
Lastly, I lightly touched on the subject in my previous response, but I have had past experience with adverse effects in using Ospho. Or rather spraying the car after the owner had applied a generous amount of Ospho on a very pitted area. My first experience with it, his buddy “did this all the time and recommended it” . At the end of the day, Ospho residue left in the bottom of the pits was a collection of acid. When he took the car to shows all that year and it sat in the sun all day, the acid residue was activated and going through its chemical process, which also means gases given off. This results in delamination and every one of those pits showed up in the form of raised circles about 1/8” or less in diameter where the epoxy primer was being pushed away from the metal.
Ever since, I have gravitated toward using media blasting or replacing rust with fresh sheet metal. It’s a lot of work in painting a vehicle and rework stings fresh in your mind despite the passing of time. So if some of my recommendations or repairs seem excessive, it is this past rework that drives my methods. While I have no plans of using Ospho in the future, when others here stress the removal of residue from pits through thorough rinsing, spot blasting, or whatever the method, take heed. Most shortcut products include their own set of pitfalls (pun intended). We as humans are naturally lazy/looking for the easy way to do something. It’s here that the allure of easy work gives us a false sense of security where someone may not clean acid residue as throughly as necessary. Rework/ repainting the entire car is a harsh lesson not forgotten.