Eryck,
Yes you need to remove glove box to get to clock and/or radio. It is fairly simple, with a couple of tips:
Once you have removed the screws holding the glove box in (leave the door completely attached but remove lower dash bolster), be careful when you pull the box out of the dash: the door stays with the box and should be in the "half-opened" position when you extract it so that the hinge and spring in the lower left-hand corner can clear the dash. Don't forget to disattach the two wires from the right side of the blower motor and fan housing and also remove the knurled nut holding the glove box light switch.
It is possible to remove the radio (this is true for stock but maybe not for aftermarket) without removing the center dash trim, but I don't recommend it. I have always found it easier to remove the dash trim (start with lower bolsters, then lower chrome strip, then center chrome strip behind heater/air levers, then radio knobs and faceplate, then dash radio surround, then pull the radio out.
But before you do anything, I would check to make sure that your stock radio has the same hole size requirement as your current radio surround. Older sl models used a smaller height opening and will not accept newer beckers. This all is up for speculation given that you have an aftermarket installed, but best to know what you have before you tear everything apart.
Finally, the older beckers used an external amp. I believe these all correlate: small opening on cars with radios with external amp; larger opening for newer sl's with radios with built in amp. Again, my only point in raising this is so that you know what you have before you tear everything apart.
Good luck,
g
'64 230sl, fully sorted out...ooops, spoke too soon