Hey Dartly,
I understand where you are coming from in addressing the structural integrity of the stock sheetmetal/subframes,. Correct me if I'm wrong but do you think that removing the factory components and replacing them with aftermarket systems compromises the structural integrity of the car? If so, there is absolutely no proof of this. There are numerous members on here using these components in everything from wheelstanding race cars to street cars (one person who commented in this thread has over 30,000 miles on his). If you can mount a gen 1 hemi to these new k-frames with coil-overs, and support the weight of that motor over the front wheels, I am inclined to think that the structural integrity is just fine. As you said, in 45 years I could be proven wrong but for now , these systems seem to be holding up just fine.