Edit . . . There won't be an actual gap, (i.e. nothing underneath the 1/4" plate). That description was only intended to indicate that there is about a 6" distance between the subframe rail and the inner rocker location.
Understood. What you describe is common. In the second pic an ancillary support for the cage is welded inboard on top of the sub frame protruding into the cockpit. First pic shows another interesting approach. It is the front pax footwell. Note the base plate is welded to the firewall. Another horizontal tube is running backward from there along side the rocker panel. That greatly stregthens the rocker panel and also can be a good attach point for additional cage members.
