This is an awesome thread!
Very timely for me to find it now, as I will need to do a cage for my race rod eventually, and would have been really tempted to fall into the "moly" trap. I was doing it to save weight, but had assumed the savings would be more than thy really are. After reading this thread, and since my main rails are mild steel, it seems logical now to do the cage in mild steel.
One question: Is there any problem with TIG welding mild steel? What filler rod is recommended? Anything special that needs/should be done?