Well I am putting a 3:42 posi out of a 75 Camaro into my 73 LT. It had a 3:08 open diff but with a 383 cu in stroker motor I could get no traction. I thought the job would be easy but came to find out that there are a few differences. I have had to change the U-Joint on the rear to a Change Over U-Joint that will fit the yoke on the pinion shaft because they use the interior clips to center the the cups and they are a bit longer whereas the driveshaft uses the clips on the outside of the cups as did the 73 differential yoke. Totally different animals. Now that this is straightened out comes the matter of hooking up the parking brakes. This is a problem as the cables off the 73 measure 33" from the end of the rear housing to the cable end where the 75 measures 22". The pedal cables are the same length but are located slightly different in the under-body. This is due to the 75 going to a "Y" type cabling and the 73 uses a "U" shape cabling done thru the use of a J hook on the passenger side to route the cable to the right rear brake. Both use a centering piece that is connected to the end of the pedal cable. The only thing to do to make this work on my 73 is to change out the cables from my 73 rearend and this is where I am today. I hope this may help someone in the future as this all changed in 1975. Cheers!