I've done fifteen pairs of arms now and never needed to resort to subframe removal.
The trick is tighten first, before attempting to slacken, which breaks the rust hold without stressing the cage securing the captive nut, and as soon as the bolt starts moving, switch immediately to 3/8" drive sockets, which allows much greater "feel" and if the bolt starts to tighten, reverse the ratchet and tighten the bolt once more.
Lots of plusgas through the hole in the bottom of the box section of the subframe, and I tend to use a mini maglite, an incandescent one not an LED so you can spray the penetrant directly onto the end of the bolt.
If the bolt is attempted to be wound straight out, it will almost certainly end badly, working this bolt patiently will pay off in as much as the resultant work of subframe removal can be avoided completely
Brian
