I just installed 6.5 inch speakers on my 91' classic. It took me many many hours, but I made speaker wooden speaker baffles to mount the speakers on. I then had to cut a hole big enough to fit the holes in the plastic door panels. Finally I sealed up the doors with 2 layers of viscoelastic (like dynamat only cheaper, lighter and blocks twice as much sound.) I then caulked any remaining holes to seal up the entire door cavity.

All of this gave me excellent midbass and virtually no vibrations. My car sounds like a Lexus now thanks to all the sound deadening (Although I did the entire car, not just the doors.)