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I would be very wary of modifing that arm as it is just a pressed bit of with bit of a lip either side giving it strength.
Was trying to find photos of metal brake arms with a bend in them, this is from a land rover defender note the thinkness of the metal . This is fitted to the land series 2 https://www.handhsurplus.co.uk/wp-content/uploads/2014/06/DSCN16931.jpg and this arm is a u shape.
Might be best to speak to people that make rally brake boxes and get a thicker arm made.
I think this is straightforward.