I have 5 1/4" round speakers in the kick panels in the front of my '74, which is where some of the series 3 cars (like my '87) also had them originally. The kick panel is plastic, while the metal behind it is steel, so cutting the holes might be tough, but because each is a sealed enclosure, the sound is very good. I was lucky enough to borrow a large power drill (1/2 hp) with a 4 3/4" hole saw blade from a friend, and the cutting was quick and painless. Only issue was the clutch pedal was in the way, so I disconnected the pushrod from the MC first. If later for the sake of originality you want the install to vanish, just pull the speakers and replace the plastic kick panel covers. Also, the speakers I used (Polk, IIRC) have a very plain black screen grille, so they look almost like a factory install and are kick-proof, too. Sorry I don't have any pics handy.