Head spinning! Seeking advice for tire/wheel fit, '66 Barracuda.

On the back a 7" wide wheel with a backspacing of about 4 1/4" should put the tire in about the center of the wheelwell. I know this from my own experience using a wheel with 4 3/8" backspacing in my 1964 Barracuda with an A Body 8 3/4" rear with the stock 10"x1/3/4" brakes. You might even be able to use just a little less backspacing. If you change the wheel width then the backspacing will need to change accordingly. If you went to an 8" wheel you would need to increase the backspacing to 4 3/4" to keep the wheel centered.

Here's a link with some pics & more info.

http://www.forabodiesonly.com/mopar/showthread.php?t=14745