We don't have synonyms for back stretch yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are backstretch.
(n) :
Alternative form of backstretch. [(athletics, horse racing, car racing) Synonym of back straight (“straight part of a racetrack, running track, etc., opposite the finishing line”).]
(n) :
Alternative form of backstretch. [(athletics, horse racing, car racing) Synonym of back straight (“straight part of a racetrack, running track, etc., opposite the finishing line”).]