Are you using OEM supplied springs or after market kit? It takes very little difference between factory and aftermarket to cause grief. You may have to pre-stretch your springs slightly to get sufficient slack to let governor hold engine speed to spec if you cannot get speed down with screw or...
You could use a thin gasket made of silicone like what they use between your truck shell and the box or one sided tape that has a ⅛" felt check in crafts shop for supply...