Which vehicles have 8 3/4 rears that are the correct overall width to fit my A-Body 69 Valiant?  

It depends on what you want to accomplish. If you want an exact match, you'll have to get a factory A-Body rear end. That can be expensive and hard to find. When doing my '67 Valiant, I just used a 68-70 B-Body rear end, and relocated the spring perches. The A-Body rear width is 51.400-inches, and the spring perch center is 43-inches. The width of a 68-70 B-body rear is 55-inches, and the width of the perches is 44-inches.

Sure, the B-Body rear may be a bit wider, but I was still able to use a factory Rallye wheel.
Also, I just noticed that there are three A-Body rear ends in our For sale forums.  

For a bolt-in replacement, find a '67-75 A-body with an 8 3/4 rear. The earlier cars, and cars with front drum brakes, will have a small wheel bolt pattern. Later cars, 73 up cars with disc brakes on front will have the larger Chrysler wheel pattern.

The last one I found I paid $200 bucks for.

Another option is to find a '66 or '67 B-body, the rear will be slightly wider than an A-body, but not by much, and the spring perches are only off by a little.

Hope this helps.