There are plenty of old English 3 speeds that would be good donors for your project. Just keep an eye on thrift stores and Craigslist.

Forence? Hmmm... Perhaps I could let you know if I find something interesting around here. Actually... I just picked up an old Mercier bike... that needs a bit of Mercy. I think it is supposed to be 531 tubing. Long horizontal dropouts (Simplex) without a derailleur hanger.

The advantage of the old 3 speeds, though, is that you naturally get the rearward facing dropouts.