At normal shooting distances, and not at minimum focus distance, the 40mm Minolta Rokkor or 40mm Voigtlander are an excellent match for the area covered by the 35mm framelines of your present M6 TTL 0.58. The lens must be modified to bring up the 35mm framelines, rather than the 50mm ones. But that is easy to do with a file! I'v modified both those lenses, and also a 40mm Summicron, for myself and others. So, you could pick up a 40mm Cv or Minolta and not have to buy another body unless or until you are ready to! But if you do get a second body, and if you bought an M3 (0.91) or an M2 or M4-P (both 0.72), then a 50mm is a better match with those bodies. But for me personally, I like the 40mm field of view better than the 50mm. SO I use my 40mm lenses on my M6, with its 35mm framelines, which are a close match at my usual shooting distances of 6 or 8 feet to infinity.
May the light be with you.