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Be careful with sizing for GB mudguards, because the measurement is from the external edge, and the internal roll is relatively large (2-3mm on each side).
50mm should work well, but you’ll need to cut and file a section out of each side. If you’re fixing it directly to the underside of the steerer tube, that should probably be okay, perhaps with a bit of extra reinforcement.
slippers-
So I had the same problem on another bike with GBs, and I just squished them in a vice. I was kinda impatient at the time. Worked fine but fucked the curve, and it sits funny over the front wheel now.
I'm not sure cutting them up is an option. I tried drilling through once, and didn't get far. That steel is tough. Had to hammer through to get a brake bridge hole on the rear.
kvragu
more than 45mm of mudguard won't fit in the fork, which is the whole problem.