The "X" bars that you refer to are #4 in the SFI spec and the diagonal bars are #8A and #8b and all have a minimum spec of 1" x 0.108" in mild steel or 1" x 0.049" in chrome moly.
The driver seat support bar is #9 and it is the one that is going perpendicular between the inner frame rail and the outer lower door bar and it's spec is for 1 1/4" x 0.108" mild steel or 2" x 2" x 0.065" square tubing in mild steel ( 1 1/4" x 0.058" in chrome moly or 2" x 2" x 0.065" square tubing).
Bill Mitchell