M_V, the references to the UN55 BB say it's JIS taper. The 7600 crankset is ISO taper. So the narrower -- 107mm vs. 109mm makes sense, though from what I've read -- Velodrome Shop, Sheldon Brown -- the difference should be 4mm -- a 105mm JIS BB. But if 107 worked for you, that's good to know.
Wildwest, my headline was misleading: my chain is not NJS, but a high-end MKS . Previously I ran the SRAM PC-1 and before that cheaper MKS. It's early to conclude, but the more expensive chain does seem bound for a considerably longer life. Anyway, they're all bushingless, non-NJS chains, I believe, and all sound about the same.
HarmanMogul, Track Supermarket's Rene has tried to help me, but the Hatta R9400, at least in my case, is not ideal. Maybe I shouldn't be such a perfectionist, but it seems that a chain line is unacceptable if it wears down the side of chain ring. I might consider the 7710 Octalink, but would prefer to stick with square taper. And I'm a bit bothered and reluctant to buy from Shimano again, since they don't see it fit to still make BBs for cranks that are not that old and remain plentifully in use.
Aniki, I bought the hubs five years ago from Track Supermarket, where they are identified as 7710. Low flange. So I have the reverse of your setup. I have measured the hub chainline repeatedly, and am sure it is 42mm, plus or minus 0.5mm.
Thank you all for your help. I've all but decided to get the Sugino 75 with dedicated BB, which is actually advertised by the company -- no other seems to bother -- as giving a 42mm chain line.