Maxcuda, I haven't ran it yet but talking to a few others who have done this this way, they claim it is much stiffer where they are part of the floor and the frame. The rear becomes a continuation of the frame if you can see the pictures good enough, if you want, I have much better pictures that show more of the install and could send them to your personal e-mail. OneBadHemi's is similar but I believe he even did more to his, you may want to ask him about it too. I'm into the whole job about 50.00 for the materials and they took about 6 hours each to fab up and install. They are .125 2X3's then I purchased a small amount of 2" strap material to tie it in in the front to make it a bit more ridgid. It was a hell of a job but I'm sure I will be glad I did it this way. As far as the carpet goes, I haven't got to that part yet but plan on just fitting it around the rails as best I can.
8 Pack, :dontknow: I am not sure how the bolt ons should fit, I haven't ever tried to install any but if they are fitting as poorly as you say, I would call Mancini and find out if there was a mistake made in what they sent you. I would think they should fit snuggly and shouldn't be that hard to install. There has to be someone else who has installed these bolt ons and can show you pics or something of how they did theirs.