As I recall, replacement hoods are supplied oversized, and trimmed to fit the opening.
I had a "junkyard" hood (bonnet for you Brits) supplied with my car that my Uncle never got around to fitting. When the restorer got hold of it, the though of resizing (yeah, that "adding aluminum" thing) a hood to fit was daunting, and he said he'd probably end up spending a lot of time (=money) and the most expedient alternative was getting a new hood (at the time this was $1,700) and trimming to fit.