The big head bullet was introduced in late 58 and sold as a 59 model. Since that bike is not a big head, I think it is pretty well safe to assume it is a 350 Bullet model. It's missing a lot of parts for sure.
If it is an older 500cc model it may be an Indian Woodsman or Westerner model or an RE 500 Bullet that was left over after production of the big head started but from the condition it's hard to tell. The engine numbers would tell the true story.