If you can find "pure" 27x1-1/4" tubes, you wouldn't have to stretch them as far as 700C tubes. Only 5MM VS 13MM diameter.
Don't forget PI! Stretch is around the whole circumference, so a 13MM diameter difference is 41MM circumference difference.
Harris lists 635MM tubes for $6.
http://sheldonbrown.com/harris/tires/635.html