now that the arms are out of the car, you need to make sure that they are not already twisted (this set was in good nick) and then make a template to compliment your measurements:
make sure the arms are okay before you modify them
here, I used paper to get my templates basic shape. because the arms are dirty, you can just put the paper over and rub over the paper, it will mark out the shape for you!
if you are going to leave the handbrake cable clip, you will need to cut around it, you can also weld to it later anyway.
after marking the template out properly ( I used a compass and drawing implements) cut it out and mark it onto the steel plate you are using. be sure to mark them the way that best utilises the material ( leaves the most usable waste piece)
here I use engineers chalk to mark where I need to cut.
I then used a guilutine to rough-cut to shape, I then used a plasma cutter to gain the shape I was after for the spring relief.
use an angle grinder with a flap-disc to clean up the edges.