I just bought a 12 posi rear for my 67 Chevelle. I began disassembling it to inspect for signs of wear or damage. It has a 3.55 GM ring and pinion in it but I am not sure that it is the original one for this rear. The ring gear has "GM" -a part number - 11 39 and 10 68 on it. is 10 68 a date code? If so it can't be the original one from this 67 housing. After removing the axles and the carrier caps the posi unit slid out of the housing without spreading the case. Is this a sign that it was not shimmed tight enough? The drivers side shim was .258" and the passenger side was .240". Is this enough total shim for this carrier?

Not sure what you mean by spreading the case. Sounds like a big no no. However the carrier should be snug if set up correctly. And not a press fit or sloppy either. As for what thickness you have on both sides that is strictly dependant on gear set up. There is no predetermined amount on either side.

RandyB..

There should be preload on the carrier bearings. The general rule of thumb is after the bearings are snug, add .007 to .010 of shim to the pass side (so the backlash doesn't change). You should not be able to slide the case out without prying. The exact amount of shim per side will vary based on many things, so no set amount will be good for all.

Spreading the case is how the differential is designed to be serviced. They have tools specifically for this task.
Von I believe the correct method for achieving bearing preload is to set the gear pattern with the carrier snug in the housing and then add .005" to each side.

The 11 39 is the teeth on the ring and pinion the 10 68 is the date code

I found information that says .484" should be the total shim thickness for proper carrier bearing preload. Moving shims from one side to the other to get
the proper ring gear backlash.

Consider this, your carrier had .498" and it was loose it should not have slid out of the housing. It should have taken a good bit of work to remove it. You will need more shims not less to preload the bearing's correctly.
Mike