fusible links?

You can't use a fuse to replace the fusible link. You need to replace the fusible link wire with another fusible link wire. The wire is only on the outside of the firewall though and runs between the terminal on the relay and your bulk head connector, the wire on the inside of the car is the main circuit.

The fusible link wire is actually one size smaller than the wire for that circuit, that is how it works, it gives the circuit a week point to burn if you have a short. A fuse will not work because the current in this circuit is not constant because your charging system goes through this wire and will keep blowing a fuse. Especially a 40 amp fuse since most alternators are 60 amp.