I had forgotten about Millett's sight set, but as mentioned, that low front sight that is made as part of the slide can be an issue. Millett went out of business many years ago and Meprolight has not made sights for the 2nd gen pistols in a very long time.
If you really have your heart set on having adjustable sights on your 659, S&W made plenty of 659's with adjustable rear sights. Those sights are the ones with the big, sheet steel protective wings on either side of the sight base. Either look for a complete pistol with adjustable sights or watch eBay. 2nd generation slides with the adjustable sight sometimes pop up on eBay.