Ken,
I just slipped under my 53 3100, and took some measurements. You better appreciate it. It is pretty hot out in my garage, this time of the year, here in Arizona. The two flat pieces that you are asking about, are 1 3/8" wide and 5" long. I am sure that you can figure out how thick you want them from the picture you posted. Hope this helps you and your sons, with your restoration. Welcome to Stovebolt by the way. Glad to have you aboard.
Rugo