That's my frame off project ^
There is an alignment hole next to the #2 body bushing on the driver's side that I got lined up first, and then I just tried to center the frame under the body at the rear. I also double-checked my alignment with a few of the mounts that were not deteriorated.
I used a plumb-bob on the ones that were too rotted out to tell where the hole used to be. I took one of the big frame repair washers and put duck tape over it and then marked the center with a sharpie. Then I held the plumb bob to the body mount so that it pointed down at the frame and aligned the washer center under the plumb bob. Then you can trace around the washer. After that, I placed another washer in the traced outline and spray painted over it to get a more exact outline of where I needed to cut to weld in the new washer.