I used a 68 B body 8 3/4 in my Mirada. It was slightly narrower than original 7 1/4 but pretty close. I got rid of the iso stuff at the same time, but it went on the springs no problem. I know on paper there's about 1/4" difference per side but as I remember I ddint have to force anything at all.
I had 275/60's on 7 " cop wheels at first but the 4 1/4" backspace put the side wall very close to the inner wheel wells. 8" wheels with a 4" backspace fit a lot better.