I'm putting together an 90's era Lemond. Threaded fork. I went to the bike shop and bought bars and a quill stem, but I'm not convinced that they match. When I place the bars in the curvature of the stem, there's a small gap at the back of the stem (ie, the top and bottom touch, but not the back). When I do this with my wife's bike (a 'city bike' with flat bars), the surfaces match perfectly. To me this suggests that the stem (and my wife's bars) are 25.4mm and my bars are 26.0mm.
I went back to the shop and they told me that the two were the same size, and clamped down the stem on similar bars without any undue effort, but I'm skeptical, mostly because I don't want to be riding along some day and have the bars fold in half on me.
Should the two curves match pretty much exactly or is a little space common?
If you are putting the center of the bars in the clamp and the sizing doesn't appear to fit, then it probably doesn't. But you shouldn't be able to get a 26.0mm-clamp diameter bar into a 25.4mm-clamp diameter stem. Unless you have a stem with removable faceplate and 2-bolt connection.
In my experience, the two curves should match exactly. Sometimes used bars have been slightly ovalized based on the clamping mechanics of the bar that they've been used with, but since both of yours are new this isn't the case. (Though you can test the ovalization by rotating the bar inside the stem clamp and seeing when it lines up and when it doesn't.)