I didn't look at the picture super close and had something else in mind.

Them's some big threads. Should take a penetrating oil pretty well. Is there access from the other end so you can hit it from both sides? Patience will be key. Get it to wiggle a bit, and hit it with oil again and let it sit over night. Assuming the threaded part is stainless and the spreader arm is aluminum, you could heat it with a torch too. The aluminum will expand much faster than the stainless.

When you put it back together. Grease that bitch.