DUBLIN'S stockbroking firms are currently abuzz with speculation about who will be replacing Goodbody's chief economist, Han de Jong who is departing for ABN-Amro in Amsterdam. The latest name in a long list is said to be another senior Dublin stockbroker. His own company, however, is said to have made sufficient attempts to keep him. Other names on the list include an economist from a smaller broker but there is also talk that the new man could be flying in from London.