The car your working with no doubt is or was a high moisture, high salt area car. This is a daily struggle in my area . I ran into this just last month on a native Kizashi and both sides were froze. Heat nut if you have to and loosen a turn. smack the nut end of the bolt with hammer or air hammer. now with a lot of leverage turn in the direction needed just enough to see it move. the bolt will still be frozen to the bushing but you will only need a little turn to make a change. tighten and recheck. Parts not only expensive but a pain to get. If you really need arm and bolt , I do have on an Arizona parts car but you would probably have $120 in parts and shipping.