It is easiest to pre drill and counntersink the holes in the brass.

There is the danger of kinking the brass so carfully bend it using a vice, pliers and a small hammer. check its fit against the stem as you go. Trying to use the stem as a bending form doesn't offer enough control to prevent un-expected bends where the bends are sharp (radius under a couple of feet)

By the way, Ive used 1/2" brass tubing for stem bandscut in half lengthwise, they work well on kayaks because the sharp ends of the boat fit into the open part of the tube.

--eric