I think you are absolutely correct about the freewheels being 13-26 and 13-34. This is a good thing, too, as I have taken to using chainwheels that are smaller than the ubiquitous 52T, as it results in a very useable high gear ratio.

If you stay in the 26T (28T max) large cog, you can use a Nuovo Record rear derailleur. If you go for the 34T freewheel cog, get a good, long-cage rear derailleur.