Spotted these wheels on a Mustang- what are they? Could they work on an A-Body?

Either look up Mustang wheels or Bullit wheels. They're are so many out there it's ridiculous. American Racing also has some variances of it (some don't look as great as others). Almost all are 5 x 4.5" bolt pattern and they come in a variety of widths and offsets.

The fronts are generally going to want a higher offset (+25 to +32ish mm depending on your brake package) and the rear can really vary depending on if you have an 8 1/4, A-body 8 3/4, B-Body 8 3/4, drum/discs, etc.?

My rule of thumb, make an adjustable wheel fixture or find a wheel with approximately a similar tire size and use the following site. I simply got a piece of cardboard, cut it into the diameter I wanted, taped a piece of cardboard to the width I wanted, fit it in the wheel well till everything fit nicely, and measured where everything was ON BOTH SIDES! Factory tolerances can move the rear end side to side under the car. My rear end was about .5" shifted to the passenger side of the vehicle.

http://www.rimsntires.com/specspro.jsp

Here are some examples:

http://www.summitracing.com/search/...st-m-black-wheels?autoview=SKU&ibanner=SREPD5

http://www.summitracing.com/search/...mustang-gt-wheels?autoview=SKU&ibanner=SREPD5

http://www.summitracing.com/parts/aew-3382-8812/overview/

Similar wheel mocked up on the back of my Dart. This was OE off of a 2002 Mustang, I believe. 17 x 8 with 275/40/18 tires. I don't recommend that wide of a tire on that short of a wheel. I'd probably run that tire on a 9" wheel, but it may have trouble clearing.