Hey there, I'm not experienced with this exact setup, just trying to help. So is your 9" lower already an overdrive crank pulley?
Also, the green belts from my understanding are pretty good, but have you tried a Gates RPM belt? I read they're not as stretchy and won't slip provided you have good tension. I found a Gates RPM 111" 6-rib on RockAuto for around $50. Part #K061110RPM. Couldn't find any 7-rib close to your desired size right now. I plan on trying an RPM belt for my Paxton charged Hemi.
If this idea interests you, you can check some other forums about them and make your own opinion. I found a few Corvette and Mustang threads with all kinds of supercharger setups. Stock Eaton, Paxton/Vortech, Whipple, etc.
Your idea with larger pulleys and spacing the A/C might help, too. I'd try the slip on solution first, then get creative.
Good luck, hope this helps.