For sure your 100% right,, GOOD CATCH! The 0.083 is absolutely for chromoly. I did a cut and paste from some e-book and then tried to clean it up,, obviously I deleted the m.s. wall and left the c.m.
Be carefull and read the book and the 2 pages following schematic, ESPECIALLY the area covering the cage for full frame car. I remember something about the placement of the additional 2 bars being what allows the 1-3/8" ,, take this with a grain of salt because I cant get into the web site from here, but I seem to remember the additional 2 bars had to attach to the UPRIGHTS, not the horizontal member of the cage. I'm not sure a "X" brace between the two main rear bars will qualify...
Your in Division 1 right, Call the head tech director Rych Cambel at 315-335-8295 (7-9 eastern)
I don't know how good Div1 tech is,,, but here in Div7 you get a call back in less than an hour usually, and an email back that evening (sometimes)
rcampbell@nhra.com
If you email keep it simple,, I know tech for Div7 would answer the first paragraph,, and that's it.
One last thing,,, your building the cage to protect a pretty valuable occupant. I wouldn't scrimp on tube size over a few pounds. The weight difference between four 1-3/8 and two 1-5/8 I bet a loosing proposition. I'd build everything but the door bars out of 1-5/8 ... but that's just me