What about drilling a hole or cutting a small enough rectangle slot in the side next to where the nuts are going into the frame
Just large enough to use a magnet and stick another nut in there with some washers
Then a corresponding bolt and tighten down like hell
Weld plates over the access...
Not made for full size car seats or large people
Only the front 2 people will be comfortable
As far as gas mileage is concerned the difference is minimal if you are hauling gear or towing versus a real v8