Just replaced the belt but it squeaks under load and when it gets warm at idle. Is it the shims /bushings nder the clutch? What do I need to do?

 

Usually loose belt will cause the squealing noise when taking off.
What is your belt deflection measurement after you put a new belt?
It should be within 25 to 30 mm with 13 lbf.
Remove a shim or two in the driven converter bushing to increase the effective diameter of the sheaves so that the belt comes tighter. Get it near to the minimum spec if possible. If it is too tight, quad may creep when idling in gear.