Mild steel will work fine. You will find most of the aftermarket xmbr's to be either .093 or .125 wall. A 1 x 2 rectangular or 1 1/2" diameter round tube are pretty common also.
Do some looking on some of the aftermarket chassis sites like Alston or Chassis Engineering.
Mark
__________________
"Bad Ast" Astro Van Pro-Touring Project.... Heat, beat, file to fit, paint to match...
Brian Hobaugh SCCA National Tour June 2014
First Hemi 'Cuda Convertible Ever Built
Short clips: Goodguys Pleasanton autocross and pit videos
Threaded Mode