I had an STW built by a former client named Tim Smith in Warren, PA, shortly after Layne Simpson published the original STW article in Shooting Times in 1991. He did one heck of a good job on the rifle for a lot less than Miller or Jarrett would have charged. I originally had my rifle built to the exact same specifications as Simpson's, with a Schneider Match-Grade 1:10 twist. It shoots 150-grain and heavier bullets like a dream; I get .33 to .5 inch 5-shot groups with two different 150-grain Ballistic Tip loads, and slightly larger groups with 150-grain Partitions. (With some different 160-grain bullets, I got very good accuracy, but, considering velocity, trajectory, down-range energy, etc., I opted to go with the 150s as my standard load.) It does not shoot 140-grain bullets nearly as well. The very best group I've ever shot with 140-grain ballistic tips is .93 inch, but most are 1.3 to 1.5 inch. I'm not sure of the factory rate of twist on your present barrel, but if you're going to rechamber the original barrel, be advised that it may shoot some bullet weights better than others.
Venado muy grande!