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Sheldon says it’s ok. Otherwise you can try and respace it.
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It's fine. When you (inevitably) break enough spokes to decide you want a new wheel built, get a 140mm hub. It's fine 'til then.
Is it a freehub style (with a cassette) or a screw-on style with a freewheel? If the latter then anything heavily laden will probably write-off the axle soon enough anyway. It won't be you, it's just the limitations of the design. Freehubs FTW!
That would be fine. JD Tandems sell 140mm hubs, that's pretty standard size for tandems.
If your frame really is 80s I'm surprised is 140mm, I thought that was a 00s improvment.