Back in the late 70's/early 80's when 6 & 7 speed freewheels & 126mm hub spacing started appearing, we'd routinely install these newer wheels in older frames with 120 mm rear spacing without any ill effects. Below is a quote from Sheldon Brown's website to that effect.
"Ideally, the frame spacing should exactly match the hub spacing. This makes for easiest wheel replacement. In practice, however, there's a fair amount of latitude in fit. In fact, when the first 130 mm 8-speed hubs were introduced, they had locknuts with beveled sides, so that you could "spring" apart the rear triangle of a frame made for the then-standard 126 mm spacing.
In general, you can safely go up one size in spacing this way, just springing the frame apart. I can't give you an absolute guarantee that this won't cause damage, but the odds are very much in your favor."
Jeff Holt
West Deptford, NJ
USA