You've only got one chance to do this.........or you're SOL.

1) Purchase a hex socket bit for the heads for use in a 1/2" ratchet wrench. Do not use an allen wrench and do not use a 3/8" ratchet wrench.

2) Get a propane torch with a small tip.

3) Heat the head of the screw for about one minute...........not excessively hot.........just enough to lengthen it slightly.

4) Quickly insert the bit and "snap" the screw with the 1/2" ratchet. The ratchet is very stiff and will apply a lot of torque very quickly.

This method should work for you. If it doesn't...........nothing will.........and you'll have to do the hammer and chisel approach as described in the previous post..........not at all fun.