I am a little jealous of your plans, but I will still offer a couple of thoughts.
If I wanted a 6 shot 686 with a lighter profile, I would get a 686 SSR. I think the 686 SSR is ugly. But the balance is nice and is very similar to a mountain gun.
If I had the mountain gun barrel and was going to have the work done, I would have it installed on a 7-shot 686 frame. Since the SSR is only available as a 6 shot gun, 7 shot L-frames with a lighter profile are very hard to find.
Whichever choice you make, I wish you luck with finding a good revolversmith and finishing your project. A 686 mountain gun is one of the best looking and best handling revolvers Smith & Wesson has made. I have no idea why they are no longer made. You will have a great gun when this is finished.