The 9 pound pulley is 2.75 that's what I got when I measured. You can order a smaller pulley from 928 motorsports
928 Motorsports - Porsche® 928 Performance Parts - 8-Rib Power Pulleys for Powerdyne, Vortech, Paxton Superchargers.
If you do I would look into a smaller belt and install a idler pulley like in this picture to get more belt wrap and prevent belt slippage.
Stillen Supercharger kit!
But once you do this I think your going to Max out your maf's for sure. I did with my set up and 3.12 pulley that comes with the kit.
You are doing good with the kit nice choice on the Meth kit nice job