Go here:
http://www.blainefabrication.com/projects.html
There are multiple shots of 4th gen bars, not drag stuff, but still you see lots of pics of how the attachment points are handled.
Basic idea is you want the plate which is fit and welded to the floor to be in at least 2 different planes. This adds considerable strength to the plate. Most shots you'll see show the rear bars landing over top of or very close to the shock mounting points in the rear. The stock floor is reinforced in this area. I don't think you need to go through the floor to try and land on a subframe connector if the plates are properly executed and the welding is done properly.