darkage,
gramps is correct. It is the brake drum, not the backing plate as I stated. Sorry for the mistake.
The best way to measure this is to get a piece of 1/2" - 1" round or square steel, and cut it to 6" (I have mine cut to 6 1/16", just for a slight tolerance) and shorten the panhard bar until it JUST touches on both ends or you can use a ruler as in your pic, just give yourself some slack to make sure you don't get dq'd by being to small.