I found this thread as I had to do this job this weekend. Here are my tips, which may be of use to someone in the future.
Clean the back of the mounting and the bolt head with a wire brush to remove the rust. Soak with it Plus gas and leave it 24 hrs.
Use a good quality 6 sided socket and give the bolt head a few good whacks with a hammer. I tried to undo the bolts but they were still stuck solid so I gave them another good soak with Plus gas and left it again for 24 hrs.
Using an extension and a breaker bar the bolt now started to move very slowly. Make sure the socket is squarely on the bolt head, if you round it you'll only make the job even harder. My shock absorbers were completely rusted in at the top, one had been replaced sometime in the past but the other was probably the original fitted 20 years ago.
I didn't bother even trying to undo the bottom nuts, a cordless angle grinder and cutting disk removed them in seconds
New shocks now fitted, job done.