The W123 will fit 4" up in the dash and 5 1/4" in the rear deck. In order for a speaker to fit nicely in the front, the tweeter must not protrude very far out, or the grille will not fit. Also, the holes are aligned like on a regular 4", as it has the euro 4" pattern. Not to worry, a regular 4" will fit with a little trimming to the metal mounting ring on the speaker.

Does your car have the "fade" wheel in the center console? If so, that could be your issue with the rear speakers to begin with. The stock grilles in the rear should fit on top of any speakers that you put in.

Any "bass" that you get from this system will almost certainly be rom the rear speakers, as a 4" cone simply can not move enough air. The only true way to add bass is to add a subwoofer in your trnk, but I have a feeling you don't want to do that.

For speaker ideas, check out Sound Domain. They are factory authorized to sell anything that they have, they are scheaper than Crutchfield, and right now if you type "BLOWOUT" into the coupon code slot, you get an extra 15% off!