Removing 1 or 2 mm of stainless would require a grinder unless you want to spend many hours with a file. Or ,as commented, take it to a machine shop. Stainless would not be expected to be hard. "200" looks like Brinell or Vickers hardness ,could be cold rolled 304, still not much compared to the 600 Vickers or Brinell of an ordinary file.
It depends on the material and slack of the wire.
A wire strong enough and pulled tight can impose large tension forces to the beam.
Say the wire is pulled straight and under 50kg deflects only 1cm.
The tension force or horizontal component of its reaction at beam will be
Fx = 280/2(1)50 = 7000 kg.
Which is impractically too large. And causes a huge moment ...