You'll need shallow mount speakers, check out Retrosound, that's where I got my replacemnts, original styles are here:
http://www.beckerautosound.com/Speaker/BMW-Classic Euro speakers.html
http://www.beckerautosound.com/Speaker/BMW-Classic Euro speakers.html
Steve,
this is very helpful information - thanks for sharing. i was trying to figure out how to locate the opening to mount new speakers at this locaiton.