I believe the blanket statement that all are cross-braced is inaccurate? I own an original owner family 3600, it does not nor did it ever have them. On the 32 trucks/cabs I have owned at least two of the 2-tons I parted did not. On Those that have it is easy to see if they have been removed because an inch or two always remains on the shroud. On GMC's they have a completely different design with a lower plate that spans both sides. Perhaps some of you can confirm if your truck actually came with them or not? I have a strong hunch that in many AD years this may have been a 3100 item.
https://www.flickr.com/photos/advance-design_parts_co/8129073652/in/set-72157631867396207
Here is a 52 3600, again original owner family with no rods or evidence of removal.
https://www.flickr.com/photos/advance-design_parts_co/15217401634/in/set-72157631870734114