Seat belt interchange
The belts will fit. There will be some head scratching though. As you know the smaller of the two retractors will attatch to the front bottom of the inside panel just forward of the rear seat. This will require trimming the interior panel around the retractor. In addition you will have to do some trimming of the metal to fit the retractor in as well. One large bolt holds it down on the bottom and a sheet metal screw holds the top. Not a big job but on the bottom bolt you will have to make some sort of plate or use a large washer. In addition you will have to weld a nut on the backside for the bolt to hold the retractor in. Going to the top of the car where the bolt goes into the headliner for the original shoulder restraint this hole will need to be relocated about 2 1/2 inches forward so the belt lines up with the retractor below as it is too far back on the 73 and will look crappy and not work properly due to misalignment. Not 100% sure and someone else may chime in here but I believe the other hole near the rocker for the other larger retractor may not be in the same location for the 75 retractor as I think it mounts in a different location as well (maybe). If memory serves me correct I think it actually would be a little forward of the original hole. Its been a long time since I did my 73 Swinger with 75 belts (WAY LONG) and I remember making it work but there was some cutting and welding. Good Luck!!!
Chas.