FWIW - The 25.4 stem arrived, and I took to it with a rat-tail file. Within 10 minutes, it fits the 26mm bars like a glove, there is no play, and when you look for daylight between the bars and clamp, there is none. With the stem cap torqued down, I can't force the bars to move. After trying, I took the bars loose and checked - there is no galling or pressure point(s) visible.

So I'm content with the 25.4 to 26 conversion via use of the rat-tail file. YMMV