i didn't use the metric rod. i used carriage bolts from the inside. bought them plenty long, used locking nuts, and cut them off to the correct length. topped them with acorn nuts.
ground off the inside of the fender until it was smooth.
worked for me.
but to answer your question, buy them plenty long and cut them off to suit you, after they are installed.

Thanks, Sugarbear. I never thought of trying to get anything in a standard size to work. Finding 10x1.25 in anything that long is hard to do. I guess I should have stated that when I posted the question, I had already special ordered some stainless 10x1.25 threaded rod. So I'm good to go on the material. I just needed the length. Installing the rod and then cutting to length sounds like a plan.. thanks.