These days, conversions don't make as much sense as they did back in the '70s and '80s. If you want a tapered barrel .41 Magnum, see if you can find a .41 Magnum Mountain Gun. However, you are limited to a 4" barrel.
If you do want to go ahead with a conversion, try Alan Harton down in Houston. He did a M28 to .44 Special for me. I did send him a 6 1/2" factory barrel though. He also did two Old Model Ruger .357 Blackhawks to .44 Special as well.
I have heard that Turnbull no longer does S&W work.
Back when Skeeter Skelton was having .45 Colt conversions done, he did run into a few cylinders that had overly deep notches. A few dimpled and one actually bored through.