that is a pretty common issue with older LA engines, especially if the coolant mix has not been maintained faithfully, The way the coolant circulates, the left side freeze plugs rot first. Then the back of the block plugs go next. When you drain the coolant, replace all the left plugs at the same time. Use brass plugs if you only want to do it once and you plan on keeping the truck for awhile. While the plugs are out use compressed air and a garden hose nozzle to wash out all the rust and sand which has piled up in the water jackets and against the plugs, causing the plugs to rot. Use that dark brown Form a Gasket (#2?) goo to coat the sides of the plug, prior to insertion .