Better to go with a longer spindle length bottom bracket or shorter? I have a 1987 Panasonic dx1000 that I am converting to a single speed. I was planning on replacing the bottom bracket. To get the measurements I measured the tube to be 70mm (2 3/4") and the spindle to be 125mm (4 7/8"). When looking for replacements I cannot find this size. Would it be better to go with a 68x122.5 mm or a 68x127.5 mm? It was a double cog and removed one so that it's only a single. Any suggestions?
I have just rebuilt a 1990 Raleigh bike (exact model unknown) and tried several bottom brackets to find the best. This turned out to be the one that brings the smallest chainring (in your case only chainring) closest to the chainstay without touching it. The one I've ended up with is 117.5mm whereas the original was 122.5mm. The front derailleur works perfectly (not applicable for your setup) and the chainline looks great, albeit aimed at the larger cassette ratios rather than the smaller ones.
My logic is that when on the larger sprockets I am putting a lot of pressure on the pedals, so a straight chainline is best for chain strength and wear. When in the smaller sprockets I am usually on the flat or downhill, so a bit of chain angle is not such an issue.
Thus, I would go for the shortest BB you can get away with to keep the chainline close to the chainstay, and minimise flexing forces on the BB stubs (are they called that?)