Say Brick. In your 1st pic I think what you have circled are the clips that hold the rear seat down. The rear seatbelt attachment point is at the little square you cut in the carpet to the right of the clips. But then I may be wrong!!
Here are some pics of the belts in my '71...
The retractor and the lap belt both attach to a bracket that went into the factory hole for the driver's lap belt...
The shoulder part went into the factory hole for the original ones that clipped up by the roof liner...This did take some modifications...
The female portion of the set went into the factory hole that is on the side of the tunnel...
I didn't want to pull the rear seat (which is a major pain in mine!) but there are 4 factory holes down at the bottom of the panel that seperates the trunk. I only have 2 sets of lapbelts (no shoulder harnesses back there) but you could put a third in the middle by doubling up on the 2 inner factory holes.
Hope this helps,
Dave