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
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"Bad Ast" Astro Van Pro-Touring Project.... Heat, beat, file to fit, paint to match...
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