Dear Pete,
I think you are nuts to worry about spreading the handlebar clamp 1/128 of an inch. Jeez, that measures out to all of .0078" ! That's less than three sheets of paper. That is as close to insignificant as I can imagine in the sloppy goddamn bike industry that I have toiled in for nigh on twenty years. About plus or minus 1/16" (.063") is a more typical "working tolerance" in the era we are fans of (LOL !). Remember - you're not working on the Space Shuttle, fer chrissakes! As with many misunderstood facets of mechanics - just because you CAN measure something doesn't mean that it MATTERS ! Many of the best-designed bike parts over the years (IMHO) share a common ability to cope with the often sub-standard and nominal sizes, tolerances, mounting surfaces, threads, and alignments (not to mention the all-too-common ignorant mechanic) that are all part and parcel of the bike world.
Additionally, many handlebars are slightly undersize (compressed) - especially if they are used. And aluminum clamps tend to stretch from repeated tightening (and overtightening) - so slightly undersize would not be unusual at all. (So watch out all you he-men with those 7mm allen keys that the list has been so pre-occupied with over the past week or so. )
Finally - the sliding clamp on the older Paramount stems is a forged Dural piece. Plenty able to cope with that small amount of "spreading" or probably even "closing down" to 15/16". If it wasn't then think of how many failures there would have been over the past 35 - 50 years that alloy stems have become commonplace amongst both cheap and expensive bikes. If you do run .938" (15/16") bars then it would probably be prudent to use some shim stock to avoid closing down the stem any more than neccesary.
A very gently applied pressure with your choice of "BF" screwdriver, or other workshop "instrument" will have your 1" bars in there in a jiffy. And you'll be on your way to burning up the boards at the next six-day on the circuit that you've hopefully been contracted to ride. Just be careful - the promoters are all crooked and their checks often bounce !
Finally, I just measured the diameter on my Paramount stem with ancient steel handlebars and the bars measure .986 "- about as close to 1" as you can expect on older bikes. The clamp looks as straight as can be and the slot is not closed down at all. If you run into problems you can always just make your own custom handlebars like Pop Brennan used to. 20 gauge mild steel in 7/8th inch diameter and then just sleeve them up to whatever y'all need. Don't forget to pack them with sand before you bend em' !
Happy New Years to Everyone,
Mike Fabian
San Francisco