So I sized up my torso first. (Before upper body) Used binder clips to hold everything in place, then made sure I could get in and out of it.
I've opted to go for a fixed, closed, torso instead of doing a split back torso.
As you can see from all the overlapped layers in the armpits. I haven't cut it yet. I marked everything so I know where to cut it and make sure the chest and back will fit over the torso.