Are metal (GB or others) guards particularly difficult to fit compared to SKS chromoplastics for example? Maybe a question for the mudguards thread although it does concern a retro mtb...
Depends on how you fit them and what attachments you use. I had to drill mine and file down stand offs. But if you’re just using normal fixings it shouldn’t be any harder.
I would recommend Berthoud. Not a retro mtb but you get the idea. I now also have matching mudflaps! They do up to 60mm, mine are 50.
I had velo orange on a rando bike and they cracked in about 8months.