I just finished a 3 link front 4 link rear conversion on a 78 cj5, moved the front axle forward 4.5 inches and the rear backwards 6 inches. Used 14 inch travel King coilovers. Took about 200 hours to complete the the conversion.
Keeping the leaf springs would be difficult unless you plan to mount the spring hangers past the current bumper locations, which kind of defeats the purpose of the stretch. Of course, you can just stretch of the frame and body. If you are keeping the leaf springs, I think you would better to get a cj7 or yj, as you get the additional length.