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

The 8.25 rear end is no slouch so whether you replace it or not is going to be answered by what your horsepower plans are. There are lots of A-bodies running deep into the 12's that run regularly for years with an 8.25. It was the heavy duty replacement for the 8.75 starting in 73 in A-bodies.

If you are building a stock displacement small block of 500 HP or less and plan to run it through an automatic the 8.25 will do just fine as long as you rebuild it with quality parts (including axles and a cast back cover). If you plan to run a manual with slicks then 450 should be the limit. A big block would have to be limited to 400 or less for the 8.25 to survive.

That rear end is likely stock to the car and is over 35 years old with who knows what kind of abuse and negelect. Don't rebuild it and it could break on your first outing but that could be said for any rear end.