It should be reasonably easy to do, and perhaps not as silly as it may sound, given that a V8 doesn't weigh that much more than a straight six.
It would be nice... nicer perhaps than a 500SL?
Given that they put the V8 in the saloon, I can't imagine it would be too hard.
You can search for pages (try googling "300E 722" for example) and find pages like this
http://www.austauschgetriebe.net/Application.htm which show the transmissions for the different models. For example the 722.4 is only good up to the 280 engine so no good for a V8, you need a 722.3 car (i.e. a 300CE or 320CE, though I'm not sure if that would guarantee to hook up to the V8 or if the other numbers after the '3' mean large or small differences.
The W124 cabrio doesn't seem to loose value either, so you wouldn't end up with a special but worthless car at the end, like you might if you did a TE Estate (wagon) or coupe.
have fun
Russell