FWIW, Mustang and F body T5s had the shifter in the same spot. The S10 had the shifter at the front end of the tailhousing.
Ok, well the question I had asked has more or less been answered. I found out that a boat transom plate I have here has both GM and Ford small block patterns on it, and they are nominally the same size. I have also found several stories of GM engines in Mercedes cars, especially the 126, but I would still like some firsthand knowledge on them. The common story is that the TH400 and 700R4 require tunnel mods, but the T5 and T56 manuals fit without mods. The Mustang does use a tailshaft mounted shifter on the T5, which would put the shifter very close to the stock opening. So, it does look like this would provide a viable option for a five speed manual in a W126. I may have to pick up another one and give it a try.