Put the stock in a vice, butt uppermost and protected by lots of towels
I bought a cheap very large straight blade screwdriver with a square shaped handle.
Put the screwdriver in the bolt head slot, press down very firmly and turn the screwdriver using a large pair of pipe grips or a large adjustable spanner , gripping the square shaped handle
Hopefully that should do it.
Remove the bolt , clean up with a wire brush and coat the bolt thread with copper grease.
Sam