Should I be pissed that it's not an 8 3/4?

an 8 1/4 is plenty strong and it has big bolt axles, with a sure grip I doubt you will break it any time soon, still not an 8 3/4 which is stronger and worth a few hundred $$$ more, if you feel you still got a good deal on the car I would not worry about it, but if you paid more because it was supposed to have an 8 3/4 rear I would go talk to the seller