There were not many W-body vehicles produced with manual transmissions to begin with. I remember seeing a couple of Luminas around the early 90s with them, but they were almost always paired with the horrible 3.4 Twin Dual Cam V6. I have driven one of those Luminas which was about 1-2 years old when I worked at a Chevy dealership, and personally thought the shift linkage was sloppy and not very well-designed. If manual transmissions were more widely available it would be a simple matter of swapping parts from a donor car, but the W-body has changed considerably over the years, subframes are even different. I always thought the Impala would be a fun car with a manual, but manual transmissions are becoming rarer as time goes on. Honestly, if you want a manual transmission that badly, it would be cheaper in the long run to sell the Impala and buy something that came with a manual. Unless you have a LOT of money to burn and are not at all concerned about resale value.