[pdxmotorhead]

You can buy a little gadget thats really for working on small engine like on mowers, its called a contact tachometer, you press it to the center of the blower input shaft and it will show you the blower RPM. Compare that to engine RPM and do a spread sheet using the pulley diameter, and RPM and just do the math upward. You should be able to calc exactly the size pulley you need. You can get the blower sheets off eatons web page which will show you the CFM of the blower at a given RPM.

Good Luck
Dave