My suggestion would be to start from the beginning with your troubleshooting.
1. Is the fuse connected to the low voltage side or the high voltage side of your transformer or mounted on a control board? (if low voltage move to #2)(if high voltage check to make sure your door switch isn't moving when you put the doors back on and shorting to the case)
2. disconnect the wires from the gas valve and put tape over the wire ends.
3. put your panels on and start the furnace.
4 it should go through the start-up to the point where the gas valve would energize and stop there.
5 If your fuse still blows then you have a short between the fuse and gas valve, ie limit switch.