If I am not mistaken the inner supports were spotwelded before the fenders were flo coat primed at the assembly plants based upon a parameter of the crest line? Amnng others I am sure.
This being the case how were replacements indexed (inner supports to fneder crest line etc.) ?
I believe it was left to the "offline" installer to create the fit required for the job..
In the plant, we welded the "shotgun" reinforcement to the fender skin prior to painting, and we "tuned" the welding fixture clamps and locators to set the crown line of the fender to match the crown of the hood.
Most Service fenders were welded (and primed) at the Regional Parts Depots, using simplified versions of our weld fixtures (which is why production take-off fenders usually fit better than NOS Service fenders).