I did this about 3 years ago to my car.....got Stockton Wheels to make them and just like yours, they had to be exact. First thing I did was move my leaf springs in as much as possible. I had a local guy relocate them and it gave me a lot of extra room without having to cut into the wheel wells. The next thing was to measure how much backspacing I needed. To do this, I ordered a tool that looks like this:
It came with some instructions which were easy to follow and it helped me determine that I needed 6.5" of backspacing on the rim so it would fit under the fender. With that tool, it has a flexible wire at the top to emulate your tire width. I bent it around my new rear tire (off the rim) and then used that as a guide. The tool bolts up to your axle (without the rim on) and then you can slide the tool horizontally to get the wire where you want it (tucked under the fender, sticking out 2", whatever the look you want), and then it has a reading on the slider that tells you the backspace needed. I believe mine read 6.25" but I wanted to make sure they fit, so I went 6.5". They fit perfect (imo) but if they were too tucked under, I was simply going to use a spacer or two to bring it out a little.
Good luck and be sure to post progress pix!
Here's some pix from what I did...btw, these are 15x10's:
