are you SURE its the belt? it could be an exhaust/intake/vacuum leak making that "squeal" sound. thats very common on the 420a at high RPM. assuming youve tightened the belt the right way and theres no fluids on it, thats my best guess.

BTW..if it is the belt, watch out for your coolent overflow resivoir. if the acc. belt comes off at high speeds it will put a nice gash into the resivoir.