I want to move the good arm rest from my parts car into the 560SEL.

I have the seat back off but am having the darnest time turning the large bolt and I don't want to strip it. I have not got it to budge!

Is it best to keep trying with a vise grip or should I get a large enough socket (my set only goes to 11/16" or so)

By the way, does someone know the metric size of the bolt?

__________________
1990 560SEL Anthracite Gray
1990 300SE Arctic White (parts)