If you are going for a SFI25.4 spec for 7.50 and slower, then all of the tubes mentioned are a problem. The main rails need to be a minimum of 1 1/2" x 0.108" MS round or 2" x 2" x 0.065" and the driver rocker bar needs to be a minimum of 1 5/8" x 0.118" MS. The rear crossmember needs to be one piece of 1 5/8" x 0.118" MS or 2" x 3" x 0.083" if it is a 4-link car, but can be 3 pieces if it is a ladder bar car. The lower rear kidney bar (the bar that goes from side to side on the main hoop at about 10-18" up can be 1 1/2" if it is a 4-link car, but needs to be 1 5/8" if it is a ladder bar car. There are a lot of issues, so I would recommend that you purchase the spec from SFI for yourself for about 35.00 and you can have it in just a few days. The specs show the different size tubing for the FC cage because of the driver being behind the main hoop.
The sportsman spec of 8.50 or slower doesn't require a FC cage, and some of the other bars mentioned, but it would be much safer for you in case of an accident to have all of the proper sized tubing.
I redid a tube chassis car a few years ago with the same problems. I just left the main rails and had to redo everything else. You can add the proper size tubes beside the existing frame rails, so you don't have to completely remove them. Here is a link to show some of the pictures of that work
http://s254.photobucket.com/albums/h...ssis%20Camaro/
Good luck,
Bill