Help!

IMHO

Your first task is to grind, sand, wire wheel, sandblast, as much of the rust off as possible both inside and under the car.

Then cut away to good metal. You can use snips, Dremel, nibbler etc. but as mentioned before BE SURE YOU KNOW WHAT YOU ARE CUTTING.

Then if you can not get repo or used good patch panels starting beating on some flat stock. A sand bag will let you beat flat stock into a concave shape.

If you don't have welding skills there is panel bond adhesives that will outlive you if applied correctly and sealed and painted properly.

Unless you are restoring a Concorde car I do t see any reason to worry about perfection just longevity.