...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...
Agree fully. Time and patience will be required.
Also, some penetrating oils instruct you to hit the object to set up vibrations so that the oil will seep into the threads. I've always thought that to be odd, but if it works???
If you do manage to salvage these parts, be sure to remove all rust and corrosion from the inner and outer threads before reassembly.
As Karl said, use grease (or anti-sieze compound) when reassembling (or on the new parts).
One last thing: On boats, especially those used in salt, if there are parts that you want to be able to remove or adjust in the future, remove them at least once a year, remove any corrosion and salt, and re-grease.
Hope this helps.
Mike