Acts of Peter
The ancient historian Josephus describes how Roman soldiers would amuse themselves by crucifying criminals in different positions, and it is likely that this would have been known to the author of the "Acts of Peter". The position attributed to Peter's crucifixion is thus plausible, either as having happened historically or as being an invention by the author of the "Acts of Peter". Death, after crucifixion head down, is unlikely to be caused by suffocation, the usual "cause of death in ordinary crucifixion".