If you use a torch, you will burn the rubber bushing out of the arm and then have to replace bushing and/or arm. If you have time, you can spray some good penetrating fluid on both sides of the bolt after taking the nut loose. Use something like K&W Knocker Loose or PB Blaster. Vibration is your friend when it comes to getting rusted frozen pieces apart. Use an air hammer on the end of the bolt. The rapped hammering may bust the bolt free along with helping work the penetrating fluid into the joint. The hammering will mushroom the end of the bolt, so you want to spin the nut back to the end of the bolt to help with a larger surface to hammer on. Keep working it with the fluid and the air hammer giving time for the fluid to work in. The bolt and nut will need to be replaced when out. Use anti-seize on the new bolt before putting it back together.
Another way to get the bolt out is to drill it out. Start with a small pilot hole in the center if the bolt and work your way up in size, drilling out more and more of the bolt until it is free. You could even cut the heads off the bolt and drill into the bolt stubs from each side just enough to pull the arm out. Once you have the arm out of the car, you can use a press to get the remaining bolt stud out of the arm bushing.
Just remember that it took along time for this rust to form and it is not going to just fall apart with some easy trick or spray lube. Your only limited by your imagination on how you go about getting it apart. Know what is stuck and where it is rusted and how the joint is rusted. This will help when trying to come up with a plan to get it free. Also a word about air hammers; if you get a cheap one, you get a tool that does not work well and is hard to control. An expensive air hammer has better trigger control and more hammering force then the cheap ones do.