[lateapex911]

My 73 has a standard one rectangular hole...known as a DIN opening.

There are kits out there to adapt the DIN opening to the old school knob style. Ebay will return things, I bet. I did some surfing, and came up with this:

230210117484 or http://cgi.ebay.com/ebaymotors/MERCEDES-BECKER-EUROPA-II-AM-FM-Radio-Stereo_W0QQcmdZViewItemQQcategoryZ38644QQihZ013QQi temZ230210117484QQrdZ1QQsspagenameZWDVW

you can see the "sleeve" that fits the DIN opening, and the radio fits into the sleeve.

And here's another setup.
http://cgi.ebay.com/ebaymotors/Porsche-Mercedes-Becker-Europa-Mexico-Radios-PAIR_W0QQcmdZViewItemQQcategoryZ10076QQihZ015QQite mZ250204098680QQrdZ1QQsspagenameZWDVW

and a last one;
http://cgi.ebay.com/ebaymotors/Becker-Mexico-AM-FM-cassette-radio_W0QQcmdZViewItemQQcategoryZ38644QQihZ019QQit emZ290197561170QQrdZ1QQsspagenameZWDVW

perhaps those might give you some ideas.