Low voltage causes a LOT of current being needed to run the starter. A slow starter is due to two things; low voltage and not enough current to the starter. You need two brand new batteries of the type rated for your truck. Do NOT get one new battery, you will kill it very quickly. What you have is one battery in two places. All of your cables need to be clean and this includes the negative and ground cables. Clean a spot on the frame where the battery negative cables go to. Shiny metal is good. New cables may be needed. Getting one of these old trucks back in operation is not easy or cheap. Do not cut corners. Winter is coming and you need a truck you can depend on to start every time. After the batteries and cables you need to look at every glow plug and test each one with a meter or a 12v test light. Buy ONLY Motorcraft/Beru glow plugs. ALL the rest are JUNK!