Wires were used instead of fuses because they would take in rush current surges without unnecessarily opening. That said, excessive current had to be present for some time for the fusible link to open. Usually there was collateral damage to other wiring at that point. If you kill the power immediately when you smell the link insulation burning, you could keep damage to a minimum.
If you do put in a fuse, make sure you have boxes of them in the car as surges will kill them and leave you high and dry.