I have a 71 sedan as well and am looking into upgrading to a 3 point system as well. Right now I am looking at getting a generic set off E bay which will bolt into the existing floor mounts. There will be a mod I have to make in order to make them work properly though.
These generic belts are a bit too short due to the fact that the shoulder mounting point on most vehicles is directly behind the shoulder. The Pinto's fixed shoulder belt anchor point is much farther back and causes the belt to be too short.

The fix requires a shoulder mount point extension which would bring it down and closer to the shoulder.
These are available on the 74, 75 etc 2 door Torino (Starsky & Hutch). Once you have these, the lap buckle on the new belts would have to be removed, fished through the shackle on the extenders then have the buckle professionally resewn back onto the belt.
Hope to have this all done in time for the spring drive. Hope this provides an option for you. I will be keeping the original belts in my collection of parts in case I ever need them.