I take it
this was the FS thread? Several weird things, one, the case for the lower gears should have the ring gear flange furthest to the right. So, your new posi should SHOULD be right! Looking at the pictures in the FS thread, it almost looks like the the 4.10s have the aftermarket spacer between the ring gear and the carrier.
So the basic design of the diffs, the higher the gear ratio, the larger the diameter of the pinion gear, the lower the gear ration, the smaller the diameter of the pinion gear. A 3.08 pinion gear is larger than the 4.10 pinion gear. So the carrier for the 3.42 and down gears would have the ring gear flange further to the left to make room for the larger pinion. I almost wonder if your 3.73's are aftermarket "thick" to fit a higher ratio carrier? But your pictures don't look like it was a thick ring gear, and if it was a "thick" set, and you truly were not using the spacer, then it should fit!
have I confused you yet?
Lets start over, are there any part numbers or markings on the posi carrier? What are all the part numbers on your Ring and Pinion too? Let's see what we have to do to get this thing set up.