Here's some reading material for you. You plan to run 10 to 15 pounds boost, on the street or track only? Fifteen pounds with a roots blower is a whole lot of boost for a street motor, especially with todays pump gas. Figure out your static c.r. then take a look at what your effective c.r. will be with that much boost.
It sounds like you have an older 1/2" pitch setup, not the 8 mm. There are pulley combos you can use to get wherever you want to be, swapping up and down, etc. There are charts in the attachments.