On an A-G body car, all of the upward load is on that upper spring seat so its good to tie it to the cage. I have seen it done with the bars attached directly to the spring seat, or in a case like yours, vertical bars up off the spring seat tying into the back bars as they pass over. Even after I backhalved the car, the rear X was tied into the frame at the upper shock mount point for the coil overs. As far as putting bars through the firewall to the front stub, they are kinda useless without all the other bars installed. You need a diagonal from the frame at about the bellhousing up to the front bar to stabilize it, and then you need to install the X bars in the door. Without the X bar, the A pillar bar isn't strong enough to stop any flex of the front stub.