It's not a fuse, it's a junction block mounted on the rad support. Power goes from the main splice to the junction block, From the junction block it goes to the battery via a fusible link. Charges the battery when running and supplies power to the main splice when the engine is off. The main splice is a big copper swage in the harness. Typically it has four wires running to/from it, One from the alternator, one from the voltage regulator, one to the junction block and another to the horn relay. From the horn relay power goes to the main feed through the firewall junction blocks into the cab.

I doubt that is your problem, but find it and make sure the connections are clean. Tell us a bit more about the symptoms of the first starter before you replaced it.