SUCCESS! I have indeed succeeded at this quest. I got the Nvidia Geforce GTX 765M from a deceased Clevo laptop and finally succeeded in getting it running in my mid-2011 27-inch Imac a couple of days ago (after letting the whole project sit for a few months). I believe that the recent Nvidia drivers included in Sierra 10.12.3 are what made it possible. Of course, I get no boot screen, brightness control or second monitor support but at least the card is running. I have made enquiries, at appropriate websites, to see if an Apple EFI ROM can be flashed into this card thereby (hopefully) alleviating the aforementioned problems. If anyone wants more info, please let me know.