There are g-body 8.5's that are a complete bolt in. Although you will need a one inch shorter driveshaft. The a-body 10 and 12 bolts will bolt in with the correct upper arms and moving the spring perches, but depending on the year they are 1-2+ inches wider. Moser, Strange, and Quick preformance offer bolt in 9 inch rears too. There is the
b-body 8.5 that is probably the widest option of all, it will require different uppers or I've heard you can drill out the other set of holes in the uppers and shorten them and it might work. And then there is the 8.8 ford rear that will work also. It gives you a really bad instant center though which baseline suspension has a kit to correct. I'm know I haven't listed every detail of all the rears but this should get you started in your research.