If you wanted an actual # of ft. lbs. that would be impossible for anyone to say for a couple reasons.
1. We don't know what grade bolt is in it.
2. An old corroded bolt won't take as much to break it as a new bolt so even if we knew the grade it would still be impossible to say.
p.s. if you don't have an impact or can't get one in there put your wrench or socket and ratchet on it and tap on the wrench/ratchet handle with a small hammer to simulate impacting. Works the same, just take a little elbow grease.