This is a familiar situation. The body of the stock bolt will have rusted and attached itself to the hole in the butt. It is not that the metal-to-metal fit is too tight. You need to make up a wide screwdriver and grind the tip so it is 1/10 inch thick and parallel-sided and preferably wide enough to just fit into the butt.
To make up mine I bought a cold chisel which just fits into the butt, and ground the tip to suit. I attached a spanner to the hexagonal shaft to get huge torque.
Others have had success grinding a tip on a cruciform wheel brace for a car.
Be sure the screwdriver tip is parallel sided or the laws of physics mean that as you twist it will exit the slot.
Rob