I just read through this, and I am considering it myself. My opinion on all the comments about seat mounting strength, and accidents is this. The seat really has nothing to do with safety/holding you in, that is all in the belts, if anything, you hold the seat in.
If I did this, I would use plated hardware, metric sized, just so in the future all hardware would be the same tools needed. And I would put silicone (sealant, not lubricant) in the hole and on each side of the washer underneath the car. I would use the largest washer I could find/fit, on the underside of the car AND on the floor in the car. I would worry about my weight flexing the floor in normal use, more than in a crash, then the belts and air bags do their job.