That's a good idea. I know a guy who had welded a few I-Beams together to build a frame bench to help him get started. I don't know if that's up your alley or not, but it seemed to work for him. We used square for the frame, but that was just because it was easier to work with. IMO the round tubing looks better. I would also get a rule book so that you can make sure you are building the standards of SCTA.
IMO I would make the car as long and low as I could. One thing on our car that I would've done differently would've been to make the frame fit to the roadster body. What I mean by that is that a roadster body doesn't just come straight back. The body kind of bows out a bit. If you make the frame to fit this bow in the body, it will give you more room to put things.
Let me know if this helps, or if you would like some different info.