This, like other MVC-specific settings, is not settable via Web.config. But you have two options:
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Override the Controller.Json(object, string, Encoding) overload to call Json(object, string, Encoding, JsonRequestBehavior), passing JsonRequestBehavior.AllowGet as the last argument. If you want this to apply to all controllers, then do this inside an abstract base controller class, then have all your controllers subclass that abstract class.
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Make an extension method MyJson(this Controller, …) which creates a JsonResult and sets the appropriate properties, then call it from your controller via this.MyJson(…).