Bean,
The RII is a track tire with a wear rating of 100. Here's the "horse's mouth":
https://www.nittotire.com/competitio...g-radial-tire/
You could make a cardboard template - basically a circle with the bolt pattern holes in it, using the tire OD on the web site. Then put it on instead of a back tire and start measuring side-to-side.
For both lateral and radial (no pun intended) clearance estimates, don't forget to account for the full movement of the rear suspension from full droop to full compression.
It's a little work but might save you a bunch of work and $$ if you do this.
Hope this helps,
Tom