4" or 4.5"

Although your question is EZ to answer there's a few options you can do, but you must be careful and understand what those options are and what you can afford. Firstly, I would suggest you read a bit of this link below to give yourself a little knowlege on the rear ends before you run out and buy something you have to re-sale.
As stated...An "A"-body 5x4" 8.75 should fit under your car nicely. If someone has a 8.75 for sale with a large bolt (5x4.5") it could be out of a B or E body maybe being "too" wide for your car. Also as stated..some guys buy large bolt 5x4.5 then have to get them cut to size as a A-body. With a swap...you also have to change the driveshaft. A lot of guys just get a 8.25 out of a A-body.

PS Yes, it's harder to find "nice" wheels for the small 5x4" bolt.

http://www.autohobbydigest.com/a-rearend.html