Using hex bits on a decent 1/4" square drive bar will give you much more controlled torque than trying to horse on a short allen key ( which will likely just bend ).
Holding the crank in place will be the hardest thing now that it's off the bike.
When it's on the bike, you can arrange things so it's held in place by careful positioning, geometry, and the chain.
You'll need to clamp it in a vice, but protected in some way so as not to damage it.
Or arrange to have the allen key clamped in the vice with a stub pointing up, and then fit the crank onto it, and apply force to the end of the crank, but the crank will be a bit short for the kind of torque you will need.