This is a fun chore. I replaced my 2 piece setup on my 73 with a retractable set from a 75 and had to modify the rear "door" panels and come up with some place to bolt the retracting mechanisms inside the panels at the correct angle and be strong enough to handle a collision ( in theory ). There are a couple of options for "replacement seat belts" but I just happened to find a set.
The mounts for the rear retractors I made are more than needed ( bolted in a 1/8 steel plate then welded then attached the mechanism. ) Sure as h3ll don't want to smash my face on the steering wheel if the sheet metal fails.