1974 Dodge van B20

Do I need to loosen the visible bolt thats under the slider bolt? Its stripping and wont come loose

there should be a bolt behind the sp/spump in the picture you are showing. The one withteh slider looks loose but the other one needs to be loosened. Sometimes a metric wrench works better than english wrenches. it's eitehr a 13 or 14 mm also use a 6 sided wrench so it won't slip unless the threads are damaged. check on the botom as there may be a slider bolt there as well but loosen the one on back. then replace your alternator belt.