Hiya Rob, well you know what Scorpion Evil is like. They use their Astro flight at 24v using 3:1 ratio on their 6mm chain. So belt drive on 5:1 ratio seems quite safe. Faster spin up time as well since the more ratio used, not as fast but who needs speed if you can keep it going.
Little Spinner is only 2000rpm but keeps hitting with the diamter and weight behind it.

So yeah Rob 5:1 ratio is a good safe choice. and can run it up to 24v if you want to.

Mr Stu