I am relaying my experience of cabinet making on how to to square things up, should apply here. I would adjust the frame frame back to get the 64 5/8 measurement down to 65 1/4. This will throw off the the two 76" measurements such that one is too long and one is too short. Then take you will need to push/pull the diagonal probably with a hydraulic ram but maybe some threaded rod will work. It will likely bounce back so take that into account. Then check all the measurements again as things probably moved (like the subframe). Repeat.

Have you checked your subframe and the body itself to make sure it isn't tweaked. If one of those is tweaked, doubtful you can get it aligned.