I'm rebuilding the back of a very old flat back and making new braces. I only have pieces of the original braces so not really great for references As the title says, are the braces/back truly flat or is there a slight arching built in...as there is on the tops of flattop guitars? And also, are there general measurements for the height of the four braces? I can come up with their width and length based on the oxidation tan lines on the inside of the back. Thanks and happy new year!