[padresag]

the mystery deepens and I hope that I am looking at this rationally. there is a problem somewhere and it does not seem to be the trailing arms but yet it could be a part of it. was this truck once upon a time a 127" w/base truck? you have just measured your w/base and you claim 114 3/4"( I am not mocking you, but just putting the facts down) which is 1/4" shorter than what the called w/base should be yet the axle is set back what where it should be sitting. there has to be a reason for that variance somewhere else. thee are 2 options here to check. 1 recheck your measured w/base or 2 there has been a modification somewhere else that is causing this.
sometimes we look and look at things and can't see them for looking at them and it is staing one right in the face all of the time. can be frustrating. truck frames do not stretch or contract on their own.
in woodworking there is or should say was an English woodworker by the name of David Pye who wrote a book called, "The Nature and Art Of Workmanship", and one of the main points that he discusses, " the workmanship of risk and the workmanship of certainty" the workmanship of risk is the person who is doing one off type of thing; the other workmanship of certainty belongs to the frame in your truck or any part really thereof. they all come out of the same stamp that makes all the frames for that model of truck.
looking at your last picture it does look like the axle is sitting back from where is should by looking at the rise in the frame. it may be possible that it is in the right place according to your w/base measurements?? that would mean then that perhaps the box has been altered.
definitely the relationship of the axle to the fender opening has been altered and it will just take careful measuring of everything involved to see what is out of whack. everybody can guess at what the problem is, but guessing from far afield usually doesn't solve to many things
sorry for the rambling but those can be the variables
ron