Yep, what I thought, awesome man.
Ok got ya, the damn terminology makes it a headache to buy / find aftermarket parts. How hard is it to call that a spring carrier BMW? I was looking for an in board bushing replacement for that arm and it gave me a migrane. It actually looks like GC replaces the arm I was thinking of with a steel joint.
I think you'll end up ahead on those 4200s. Moment of truth will be after you speak to whomever is re building and valving.
If your able to get the car corner balanced / weighed it will make spring selection easier but let us know what you decide on