The speakers on the dash are very easy to take out, they have 1 or 2 small screws on the grill. Once you pup the grill they have some scews around the speaker and that is it. The stock speaker wires have some fancy conectors you have to detach before you pull the speaker out. I don't like to splice wires together so I got some new conectors (if you buy from Crutchfield.com they come with those connectors). I don't know how much $$$ and effort you want to put into your stereo system, but fancy speakers of the stock dimensions will not do much to improve the sound. If you are not going to use an amplifier I wouldn't spent much on speakers.
I used to have a '79 280E and upgraded the speakers and stereo nothing fancy a clarion ($300 cd player) and some newer speakers of the same original dimensions and location (i think 3 inch infinity Kappa in dash and 4 inch Kenwood in back). They did made the music a bit better but definitely needed some bass, and for that you need bigger speakers or a sub. I bought an amplified Pioneer tube and that did the trick. I had to put it on the floor behind the driver's seat because the bass would not go through the trunk (because of the fueltank and rear deck).
I am sure you can buy some kick panel enclusures similar to the ones on the picture above, or cut out the rear deck to attach bigger speakers or a small sub.
SCLJA - That is not the stock speaker right?
I hope this helps...
