OK, I think I understand WHY now.
The wrong battery is in there. The positive (+) post needs to be towards the center of the truck, but it should be to the REAR of the truck, not the front. Your battery cable is too short (I don't see the cable going to the starter)
Then you can get rid of the PO contraption and additional connections.
Install MIL-spec battery clamps and attach the things as they need to be attached.
Don't cut and hack on your Fusible links, as they are designed to burn up and fail to save the rest of the truck. If you alter their length, connections, etc., then they may not perform as designed. In this case, I would think they would fail early, causing you serious heartache to have to work through this mess again, on the side of the road.