It’s just center distance between your two pulleys. Shorter is better just as long as you have enough teeth in contact.
Our c-clip is recessed on our motors. I’ve ran motors with the c clip being too big for the center hole before and it works fine and possibly the c-clip can get damaged but there’s not much you can do.
As for the clamp, you’ll need to fit a piece of the heatshrink on the curvature part of the caliber truck. This makes it more solid. Don’t rely on just the M6 bolt without a heatshrink. Hope this helps. Please let me know if any issues.