The frame rails on the late b are essentially flapping in the breeze. I think your boxing the radiator support is a good idea. I'd also suggest doing a pipe from the firewall down to the end of the frame rails to triangulate them and to stiffen them up. I wouldn't make them straight. Not only would it interfere with the bolt on inner fenders, but a kink would also give it a crush zone in case, God forbid, a front end collision. Solid iso biscuits are best, but because of their thickness, the K-frame's loads will have a larger torque vector on the frame rail rather than a non-iso mopar, so it won't be doing as good a job as the other Mopars. A late B K frame is mondo heavy compared to the earlier ones, too. There's ways to lighten that up without sacrificing stiffness (think installing windows, but welding up the trim) but now we're in mission creep regarding not going max-effort. A Monte Carlo bar from the firewall to the top of the fenders is almost worthless as there is no strength up there on a late B.
I wish I was more knowledgeable about "late" B's. A lot here to digest. But I will mention a minor detail, adding a straight "pipe" from firewall to frame rails, only becomes a "spear" IF a load is applied on axis to the center of the tube, nearly impossible, and adding any bend to allow it to collapse in unforeseen circumstances, obviously significantly weakens its stiffness, which is then overcome by increasing its dia/wall thickness, in a vicious design circle. I solved this problem by using CF tube, it just shatters. IMO, most race cars put a bend in the forward down bars mainly for fit. The orientation, again IMO, becomes more critical for safety issues the more horizontal the bars(s)(any) become.
