Hi all,
I'd suggest more research on the Green Drake. It is my understanding that the flies are very quick to hatch, and begin doing so well before they reach the surface, springing through the film fully formed duns. It appears from my observation that this is true. A transitional flymph or soft-hackle might work better than a nymph pattern.
Mark
http://libstudio.com/FS&S/html/green_drake_emerger.html Here's a pattern I created some time back. It could use a little tweaking- perhaps one of those nice Grip style emerger hook, real pale silk thread for the abdomen, but I think, essentially, it's a good bet which has worked for me in the past.