I would guess one of 2 things. Either something wrong in the ignition switch or a dead short in the wire going to the starter. I would first unplug the ignition switch. Figure out which wire goes from the ignition switch to the solonoid. Get about 10ft. of 12 gauge primary wire. Hook one end to the + side of the battery.
Strip about 1/4 inch insulation off the other end of the wire and carefully take it inside of the cab. Touch the wire that goes from the ignition switch to the starter. If the engine cranks, you have eliminated a short in the wire from the switch to the solenoid. If it pops and throws a heavy spark, you a short in the wire somewhere. Quite often the grommet is missing where the wire harness goes through the firewall and the insulation rubs. You could have wiggled the wire just right to cause a short. If you have no short in the wire, plug the harness back on to the ignition switch. Disconnect the wire from the ignition switch from the solenoid at the starter. Now try moving the key to the start position. The gauge should only show a slight discharge from the coil. If it pegs the ammeter, something could be wrong internally in the starter switch. Before you replace the starter switch, buzz out (that is test the wire with your hot wire from the battery) the wire that goes to the coil and make sure it doesn't have a dead short. My guess would be that you are going to fund a short in the wire going from the ignition switch to the solenoid.
