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

Your next move should be a B or C body 8 3/4 in my opinion, preferably the C body, for the bigger brakes and it's cheaper. Even if you want to stay stock wheelwells and rearend width, or mini tub, you can have the housing and axles cut to whatever dimension you want for pretty cheap - probably cheaper than buying an A-body 8 3/4.