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

i wouldnt be mad at him. you need to know what you are buying, crawling all around the car when you look at it. you cant take anyones word for it. just use it as a learning lesson. there are guys out there looking for 8 1/4's maybe you can sell it to put money towards your 8 3/4. if you get a stock 8 3/4 though, you are going to need to switch it to big bolt pattern to match your front.