I personally think they are a waste of time on a B/E-body car. Certainly the price outweighs the advantages by a large margin.
The Alterktion crossmember was developed to make it much easier when building a big block A-body such as the Dart.
Headers have always been the main problem with this engine swap so the Alterktion kit is a Godsend to people wanting to do this.
If I had a Dart and wanted to fit a 440 or Hemi I would have no hesitation to buy the Alterktion crossmember.
The only advantage on a B/E-body is the addition of R&P steering.
However the advantage of R&P steering over stock manual steering is minimal and for the cost of the Alterktion conversion it is not worth it.
They circuit raced the AAR and T/A with MoPar manual steering so it can't be that bad for a street car can it?
Also remember that the stock MoPar torsion bar suspension is a very good (if not one of the best) suspension system. Changing to coil-overs is a step backwards in my opinion.
The crossmember kit may be less weight in total on the car but I doubt the unspring weight is better at all. Probably heavier. You would be better off fitting lighter wheels.
Headers are no problem on B/E-body cars either.
So on balance I think if you own a B or E-body car you would be better off spending that amount of cash on something more usable for the street such as low rear gears and a GV overdrive unit. Or maybe a ProCharger kit.