How tall is it going to be?
I use 5/8 on anything taller than a short loop.
If it's going to be used to strap shit to or a back rest, I'd use 5/8 and not worry about it.
My BSA is rocking 5/8 and it's solid as shit. I could put a fat chick back there no problem.
Getting this shit chromed and painted this week....