I went through this recently with an '85 Cannondale SR300.

Initially I was using the original 6 sp. double derailer with a used Campy 9 sp. triple and 7 speed freewheel, with a long-cage rear derailer. It worked as long as I slightly over-shifted up off the granny then quickly brought it back slightly to the middle.

It only took accidentally shifting to the granny, then not being able to get off it on a medium hill once, to make me buy a triple derailer. I got an inexpensive Shimano Sora and have had no trouble with it.