There is a E.T. limit on using mild steel tubing for SFI cages, I believe it is 7.50 or slower in the 1/4 mile If your planning on keeping the car and making it goes faster and faster then go for the good stuff to start with As Mike has pointed out it is stronger and lighter as well as returning to its original position in race cars during the runs instead of bending and staying bent after the stress
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)