A 1977 Ford pickup will have multiple freeze plugs (also called core plugs). They're located in various places in the engine block and transmission. There isn't one single location. To find them, you'll need to consult a repair manual specific to your truck's engine (e.g., a 302, 351, 400, etc.).
A repair manual will show you diagrams and precise locations. Without knowing your engine size, I can't give you specific locations. They're typically found on the sides and bottom of the engine block. Some might be hidden behind other components.
