I have an Atax stem ('85 Peugeot). Original steel handlebars are 25 mm.
Is it prudent to spread the stem to accommodate Nitto 25.4 randonneur bars?
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I wouldn't try it.
70's french stems aren't trustworthy, 80's don't seem much better and weakening a stem isn't exactly worth the risk anyway (I'd opt for sanding it rather than spreading it to remove the necessary material instead if I had to, but I'd recommend getting a new stem instead). You'll probably scratch up your nice new bars too.
Try to get the current stem out and replace it. If not going to come out, you may want to try sanding it, but I'd recommend just living with the existing bars.
Why bother? It's much easier/cheaper/faster to get a replacement stem that can accommodate contemporary handlebar diameters. I replaced the AVA stem (nicknamed the "Death Stem" by Sheldon Brown in the link on Batman's answer) on my 1970s Peugot with a Velo-Orange stem and bars.
What you need is this: Velo Orange Threadless Stem Adapter (ST-0001; $16) -- this will allow you to use any modern set of bars with your old forks.