A fusible link is better than a fuse and there should already be one at the starter relay. If it burns you should replace it with another link not just a wire.

If you go with a fuse get a large fuse like you would use for a stereo amplifier and start small around 30 to 40 amps. If you look at all the stuff these old cars had there wasn't a lot of current draw. Most had no power windows or seats etc. Lights, ignition, radio, AC/heat and wipers, PRetty much sums it up.