When you go to buy it there will be the thread size and the OD stated in the info. Just measure where you want to cut it and get the one closest to that size. If he has a lathe he can blend it in so it looks like factory. Are you going to have him pin it so it will be easy to change if you ever want too? I want to do mine if I ever get caught up.
The man that die's with the most stuff win's!