Just take it one step at a time.
You weren't using the starter, The ignition switch was off, The battery wasn't hurt.

Alternator is toast, on the rare chance it had an external regulator, it's gone too.

You will have to replace a couple of fried wires. Make sure you check carefully that those wires didn't burn through adjacent wires they were bundled with.

That is about it.

Disconnect the wires to the alternator. If it is a single wire type you are really lucky.

Remove the burned wires from the truck. This will allow you to find any collateral burn damage.

Leave the alternator in place for now and get back to starting the motor.

If it starts and runs well, you can buy a new alternator and wire it in.

If the motor has problems, you don't need an alternator.