To all the posters on this thread,
Another approach to consider is simply to set up a freewheel with larger cogs across the board, resulting in more or less the same tooth difference as a more standard 14-26. I acquired a Mondia that had been set up this way and it works really great.
Granted, you would need access to spare cogs, especially if you want to keep everything true to period.
Nobody, in my opinion, needs a 52-13 combo if they also need a 42-28. I don't think anyone doing recreational riding needs anywhere near a 52-13.
my two cents.
Bob Schenker
Oakland, CA