Your car stock should have a 3.5 ratio 7.25 rear. Thats what someone swapped into my car. Also, that same year they had V8s in your same car so that means they have 8"s that will fit. Look for an early 8" for a easy swap, no mods except for the new v8 yoke and possibly the brakes but all simple bolt on stuff.
Your car with an auto came likely with a 3.1, with a manual it should be a 3.5. Which means you're turning about 3k rpm With 25" tires. Slightly higher like maybe 50-100 more with 24".
The prob with a 9" is that they're very heavy and overkill for a l6, especially for a 144 or 170. If you plan to put a spooky hopped up 302 or something then the 9" is the way to go but the price will be a lot of work to shorten and crap.
Your car came with either a 7.25, a 7.5 (For the Rancheros and maybe wagons, not sure), or the 8". Stick with something that was made for the car and it will make the process much smoother, cheaper, and easier.
Also, to check if its a 8" or the 7.25, check the rear of the diff. If you have bolts on the back and can take a plate off to access the guts, then its the 7.25 or 7.5, if not and the access is from the front and the hole carrier has to be pulled, then thats the 8".
I did this swap from a 7.25 to an 8" and it wasnt hard but it wasnt great. It was $140 to cut the driveshaft, weld the new yoke on, and balance it, thsts cheap. Plus i paid $250 for the rear. It came with new 10" brakes on it but the wheel cylinders were dried up and nasty and it needed new auto adjust linkage, so about $40 for the new cylinders and $20 for the new auto adjust stuff. Then about $120 for ebrake stuff since it didnt have it and the original stuff wont work with it.
Good luck man,
Ryan