The place where I purchased mine had a adapter ring to compensate for the depth. With the ring you can put in a nice speaker with no problem. Check with a car audio store. I went to Concept Audio in Angola, IN.

The better quality speakers don't rely on a huge magnet like they used to. Newer technology allows them to be much less depth than before and still deliver outstanding sound quality.

Check with a auto stereo shop that carries brands like Boston Acoustics and Infiniti. You should be able to find something.

Check out crutchfield.com. They show that everything for the rear of an 80 is a modified fit. Give them a call, they just might have the spacers you need to do the job. I've bought from them before and I've always had good luck with them.  

i just sort mine out, get catologs, all seem to do models in right size.

If you have the original head unit, I understand that putting the 4 ohm speakers in may damage it as you have a 8 ohm or 10 ohm head unit. I'm no audio expert but you may want to check it out.