My '64 has the amp mounted to the firewall, above the passenger footwell, oriented vertically. It is invisible from any view. I am fairly sure it is the original mount, which is a large wing-bolt that threads though the hole in the amp and fastens into the threaded hole in the firewall. It is a huge pain to mount because of the blind hole. You might want to check first to see if you have the hole in the amp and the firewall.
Best,
g
'64 230sl, fully sorted out...ooops, spoke too soon