Need to identify GM posi trac

[Blazer73wip]

I bought a positrac unit that was suppose to fit a 12 bolt GM truck differential. The unit will not fit in the 12 bolt housing, too large. The unit is too wide to fit in the housing without any shims. The unit has 8 7/8 ring gear, same as the old unit. The casting numbers on the unit is 249 and it also has 1105 cast in the housing. We are trying to upgrade a 1973 Blazer, with a posi trac and 3.73 gears.
Any information would be appreciated. I have photo's of both units if needed, just don't know how to post them on this site.

 

[sweetk30]

new members have to link pics from sites like photo bucket.

hard to tell with no pics tho.

welcom and good luck.

 

[tRustyK5]

Possibly a car 12 bolt posi unit you got?