I just went through this exercise a few months ago, and it's a bit of a crap-shoot. You can thoroughly measure everything, but it's still hard to know what your car will look like once the wheels and tires are on it. I have a '67 Barracuda with 8-1/4" rear and front discs from a '73-76 A-body. I think the rear is the same width as a stock 7-1/4" but the disc spindles push the front wheels out nearly 7/8" on each side from their stock location. I don't like that look, so I used a different backspace in front, to pull the wheels in a bit. Using 14" wheels (as I did), some problems you may run into are availability of wheel width and offset, and also tire sizes. My wheels are Large Bolt Pattern Rallye style from Specialty Wheel, on BF Goodrich Radial T/A's. Here are the sizes:
Front: 14x6", 3.75" backspacing, 205/70R14
Rear: 14x7", 4.12" backspacing, 225/70R14
I think the front tires were still available from Goodrich, but not the rears (in that size). My tire shop obtained them from Coker tire.
There is still room between the rear tires and leaf springs, and I think I could have used an 8" wheel if I wanted. My front torsion bars are adjusted down slightly from stock. The rear tires are about 1" larger in diameter than the fronts. I wanted a slight rake, but nothing too radical. Plus the Barracuda wheels wells are huge, and I wanted to fill the rears a bit more. I am happy with the end result. Just take your time, measure everything, look at lots of other cars, and check availability of all tire sizes, wheel backspacing, etc. before ordering anything. Good luck!
