I just finished installing a set of these brackets in a 71. The rear bolts are the same the front bolts set inboard 3/4". The hump in the bracket goes to the front, and sits almost to the edge of the floor reinf. Take a chaulk line and string it from the rear seat belt holes from side to side to help make sure your "square". BTW the brackets are not user friendly since they need a bow in them. I had a heck of a time getting them to line up. I then drilled small pilot holes through the bracket & floor and inserted a drywall screw to hold it into place so I could weld it in.
Jody
Jody