Hi Scott,
My '28 Roadster has a Trunk but there was evidence that someone had attempted to convert to a rumble seat, and it was an absolute mess. The door, hinges, latches and panels above and below the door are all different. If you must have a rumble seat I would reccommend you buy the entire back end off a body that has one and be prepared for a bit of meticulous refitting. I also attempted to convert my '28 Coupe to a Cabriolet in 1948 and it was even more of a mess, doors flying open, nothing fitting, etc.
Herb