you might check your seat tracks to make sure they go fully to the rear, if they do modify the mounts or floor holes to move the seat rearward, Lawrence
They required a bit of coaxing when I first got the car, but they are
Modifying the floor is out of the question. The floorpan is recessed in one spot (per corner) specifically for the steel mounting reinforcement that is located under it. That reinforcement is welded to the rocker panel. Any modifications to that would require hacking up half the floorpan. Too drastic.
The rails are very compact and would not appear to be easily modifiable, though leaps and bounds easier than the pan.