My approach to it:
I cut out the rockers and grafted in 3x3, 3/16" wall tubing....kinda' boatsided since they are raised up to the bottom door hinge (had to cut 4" off the bottom of the doors). The original (rusty) floor was cut out to the trans tunnel. I then ran 1.5", 1/8" wall square tube between the rocker and the original body "subframe" near the body mount locations.
2" angle was also ran between the subframe and the rockers for the seatmounts. The new floor is 1/8" plate......I used the 1/8" stuff so I could incorporate the floor as part of the overall body support (versus building a floor support and then just skinning it with sheetmetal). The floor is welded to the what is left to the original floor on the inboard side, the original subframe, and directly to the rockers on the outboard side (I had to slot the inner sections of the A and B pillars to due this, and then rewelded them to the floor). The seats are currently bolted right to the floor, with the two outer mounting bolts also going through the above mentioned angle iron support.
The cage, which will be started this weekend, will be bolted and/or welded to the 1/8" plate floor with reinforcements coming from the rockers to the underside of the floor where the cage mounting points will be.
For right now I will be using the stock seats and seatbelts, but will hopefully switch to harness's and racing seats next spring. I'm thinking with this design that I can keep the seat mounts and lap bolts mounted to the floor, with the shoulder straps on the cage since everything is directly tied together. For the seat to come loose from the body, and the cage, it would have to rip both the 1/8" floor and angle iron supports off the rockers........if that happens I think there will be more to worry about.......