Depending on the condition & size of the engine and several other factors, a starter can pull 80-350A, and as the battery gets weaker, the starter will pull MORE. But a bad starter can catch fire pulling less current than a good one so that's why there's no fuse: the circuit isn't defined by a maximum normal load, so it can't be protected from overload. It's just overbuilt so the starter is usually where all the bad things happen, and you change it out as a unit for any failure.
The fusible link wires ARE the protection for the main power to the truck, including the alternator (which should have its own). If you want to replace them with fuses, that's fine. Place your fuse block as close as possible to the battery.
The size of each fuse depends on the size of the wire you connect to it, and what's on that circuit. Any fuse or fusible link should burn out at ~110-133% of the maximum acceptable load on the circuit, OR at <50% of the current necessary to make the wire warm enough to notice (whichever is LOWER).
