1. Buy the tires and wheels that you want to run
2. cut out the wheel wells and anything else that is in the way of the new tires
3. put the tires up in the wheel wells and set up your car at ride height, or block up your tires at ride height if the body is off the ground or front sub frame.
4. start measuring and planning your build from there
5. before you pull out the new tires, take the measurements for the new axle width. If you are uncomfortable with this step, call the company that you will purchase the rear and / or axle shafts from, and get directions from them about the proper measurements that you need to supply to them.