6 Rib will have a lot more parts availability and belt lengths. But may slip under lots of load at high rpm.
What size is the magnuson blower and what size pulleys are you running?
The bigger you can run both (crankshaft and blower) pulleys the better, it essentially increases surface area and mechanical leverage on moving the belt, and the only downside is increased belt speed. Which is an excellent trade-off
I went with the 10-rib since that was what the factory used on the gt500 for this blower.
I went with the cummins ISX tensioner after looking at prices for an aftermarket gt500 blower tensioner and deciding that anyone who spends $400+ for a shiny billet tensioner needs their head examined.
I think I got the cummins unit for $79 USD at the time and it probably has 50% more tension and will live twice as long as their shiney one.
