I think Josh Smith would be a great fit on this team in the long run, and I think he's a better player than Jeff Green. I don't think it's at all likely that he gets traded to Boston for Jeff Green for a variety of reasons.
The best chance for Smith to come here, imo, is if PP decides he's retiring after the season, and agrees to be traded to Atlanta in an S&T deal. I don't find that especially probable, but it's more likely than Atlanta wanting a grab bag of salaries that don't match up well enough.