On my '82 300SD they are kind of recessed. I believe I used an extension and a 10mm hex socket to get my off...could have been a different size, but I do know mine were hex. If someone can't provide a better explanation, I am actually putting my new engine in tonight and have to put the bolts in, I can take some pictures. I am going to assume that the process is similar between the two body styles...

Oh, and there are two bolts, one on each side that bolt up into the metal engine mounts that come from the engine. The upper shock mounts need to be removed as well, but they don't hold th engine in place at all.