attach it to the bumper in the front, then at the rear go all the way back to the rear bumper, or just extend a mount from the frame, and weld it to the mount.
Exo cages dont tie into the body, they go around the exterior of the body;hence, why its called exo. If you were to tie solid tubing into sheetmetal the sheetmetal would most likly stretched or crack.
From what I can see you will have to attach it to the frame, whether by welding, or welding a plate to the tubing, then bolting the plate to the frame. Or you could built mounts simular to body mounts, but extend wider and weld them to that. It has to attach to the frame somehow to be sturdy enough.
A few weeks ago there was an exo cage thread, look at the pics to see how it goes together.
Good luck Dale!