I know those extended seat stays were a GT signature feature, but that is all I know about GT bikes, but Henry James DOs? I believe I only saw those on more special bikes. The DO connection/brazing detail looks really nice too. I hope you still have the original fork for the bike as it must also have nice deails on it.
Someone more familiar with GT bikes should chime in soon to give you more info on your frameset.
Chombi