You know what, its a unibody car. The frame you keep spouting off about is probably a 16 gauge channel spot welded to the floor pan. The job is going to require removing at least the seats and carpet, and may involve a consel and dash removal because any fix is going to require welding to the floor pan.Depending on how far up the firewall the repair will have to go, there may also be wiring that needs to be removed.

If i had to remove all the interior, I would do the repair from inside the car, one side at a time. Zip cut the floor and the damaged frame rail out of the car. Fabricate a new frame section to replace the rusted area (I would use an 1/8" wall square or rectangle tube if the size was close) and weld the new frame rail inside the original frame. Then weld a patch onto the floor pan. Repeat the process on the other side. Seam seal and paint the floor patch, replace the interior.
From under the car, if possible, seam seal the edges and paint the repair area, then add the late model heat shields.

The repair area will always be there. Structurally, it will be nearly as good as originally designed, and infinitely better then before the repair. The repair will probably out last the car.
No need to pull the motor. There will be as much time in removing and replacing the interior as the repair will take. I'd do it for that $1150 estimate someone else first suggested, and I'm not scared, I've actually repaired cars that were rusty. If you want to save some money, pull the interior before you send the car here.

After I do the first one, the price might be adjusted up or down. I'll be waiting for the LX Platform cars to be lining up on the street in front of my house. Gene