From here it looks like for bigger tires the body may need to shift forward. On the fender lips a guy could sort of split the different angles of the current rear halves and use the angle of the back window and trunk. Semi-streamlining for a less stodgy, more unified look. Just thinking out loud and thinking thats probably like 13" rubber in the pic.
Your project is a cool adventure, should be a lot of fun in a little package.
