I was comparing a '76 1/2 ton ford frame which is under it now and it is 5x3x.187 channel. There was a possibility of a > '97 dakota frame and that is 6x3 rect (I think). Didn't the 427 cobra use 4" dia x .090 tube? I don't plan to do any heavy hauling with it and I think that's why the truck frames are over built. If I were to buy new steel, I was leaning towards 5x3x .187.
5x3x.187 comes in @ 9.68 lbs/ft and 5x5x.125 is 8.27 lbs/ft. Seems that I could have a larger cross section and greater torsional rigidity and lighter weight. Since the sub frames are complete units, I don't think that there will be a lot of concentrated dynamic stress points where the wall thickness will be an issue.
Point is that steel ain't cheap and I already have it.