The SCCA now allows welded connectors on all but Stock Class now for Solo Racing. Anything through the floor puts the car into the Prepared category (CP for RWD Mopars). Some other things I'm doing at this point, also put me there, even though this is being built as a "beginner" car where I can upgrade it as time and budget allow, but I'd like to get the foundation right first.
I've heard of the X design. From a structural standpoint, I can see that it would be much stiffer. Its also legal in CP (which at the national level have become full cage cars). That cage of yours is a great idea. Especially about tying in the XV fender supports (those and their radiator brace on my wish list). I was thinking about running a horizontal bar behind the firewall tying the two together underneath the dash, and then two verticle bars tying them into the floor plates for the Roll Bar's down bars. Yes, I was trying to avoid a full halo roll cage in this car, but maybe that's a better idea (it probably is, I just have to buy the add ons to make the bar kit a cage kit). As for tying it into the body, I don't know if I want to go that far for fear of the challenger's future.
K and torsion bar gussetting, duly noted. I knew the K would need some help, but I would've overlooked the x-member until later on when you all would see that classified ad looking for a new one.
BTW: between yesterday and today, got a bunch of tracer lines on the bottom of the car, haven't cut anything yet. Got to remeasure the subframe depths and see what that's going to do to the notches. (and HP2 I did see your other thread about using many jackstands or a frame jig, I wish I had the room for the jig, but I just ended up buying a couple more pairs of jackstands and going that way)