seat belts?

Using your photo above as a reference, you’ll have to remove the short screw shown in the LH corner, the two longer screws at the opposite end of the piece, the vinyl-covered kick panel below that held on with spring-loaded wire clips (there are inexpensive special tools made for this) which will expose 3 more screws at the bottom of the plastic piece. The top of the plastic piece has a long ‘L’ flange clip that is metal and is riveted to the plastic - it goes into the groove between the pinch-weld and the weatherstrip and 1/4 window. Once all of the fasteners are removed, start at the riveted-on clip and lift up just enough for the clip to clear and slightly roll out/twist the top away from the 1/4 window. Now go to the opposite end of the plastic piece and ease it out - it should come out. That riveted-on clip is the major PITA in my opinion. Installation is the reverse of removal. 

It goes without saying to try not to break this part - there used to be repros available but I’m not sure if they still are and if they are they’ll be pricey. And like all repros, they’re not 100% identical to the originals. 

Steve_P wrote: I think he wants to install a modern set with an inertia reel in the shoulder belt.  If his car has a shoulder belt, IIRC required after 1/1/68, there is no cutting, welding....   it's a bolt in.    So what is a good retrofit?

Steve,

Yes, you are correct.  I want to install a modern set with an inertia reel in the shoulder belt.