I recently looked under my new purchase while pulling the fuel pump and filter. (1981 Spider 2000) I am pulling the hard fuel lines to replace with SS 5/16. I found the rails under the seat rail and frame was frayed and hanging down like rusty moss. I could look down between the seat rail and the sill and see grass. How hard is this to replace and what is the best option? I will try and post pics of this soon.

This is a very common area for rust. Best repair would be to weld in new metal. It does not have to be pretty, as it is not an area you see. I recommend welding because the seat is fastened there, so it's good to have it solid.
You could also neutralize the rust and use fiberglass, but I never liked the idea of foreign materials in steel bodies.

If you scroll way down you can see how I repaired mine:

http://italiancarclub.com/csaba/124body.html

Nice job, and nice car!