I don't know if this is the answer you are looking for, but it's all I got: I was over where my parts 440 is and looked what it had - all original, never crashed....just dissolved from the doors back thanks to the great mid-west salt lick. It never had bumperettes/guards front nor rear. It does not have that cross-brace. I neglected to look at the front chrome to see if it had holes punched where the top of the guards would mount - will try to remember to look next time I'm over there; "guardless" rear bumpers don't have the extra holes, but the fronts might as they would be quite hidden. I doubt that any guardless 70 Coronet would have that brace from the factory, but I don't know that as fact. Mine's a 500, so it has both front and rear.
Had one additional thought on this (and I think you were basically soliciting opinion?): I doubt you'd gain any significant strength by using just this (and if that piece is absorbing impact force, you've got bigger problems than just this piece! LOL), so if you're not using the guards, then sell this to someone that needs it for there "guarded" car? All of these oddball one-year-only Coronet parts are intrinsically valuable to someone out there doing a build/rebuild that would use them. And you've even got both the adapter wedges and all the fasteners!