Dude, I wish that it was a nut on the stock bolt. It requires a flat screwdriver. I had to go to Tractor Supply to get a 10" flat driver. I just can't get enough torque with it. Part of the problem is that the slot is about twice as wide as the tip of this driver. Going to try kerosene or some kind of penetrating oil from the receiver side when I get a chance.
I've had good luck breaking seized nuts and bolts loose using Kroil. Its tauted to be the best penetrating fluid available. I've seen it at well equipped gun shops. I think Cabelo's sells it, as well. Brownells sells it also. A little goes a long way.
Years ago I purchased a set of large, over-sized screw drivers. I call them "mechanics drivers" because I've only seen mechanics use them. They have thick, long shafts and large handles. Unfortunately, they have tapered blades but they will fill the slot on stock bolts pretty well. I've had some tough bolts on a few Enfields, but so far the big screwdrivers have done the job. The large grip and thick shaft allows for the application of a decent amount of torque.
Don't know what a set will cost today, but perhaps you could visit a local service shop with your butt stock and ask to use one. More than likely, if the shop will allow it, the fellows will probably want to help.
Good luck.
