I had to do the same thing on both sides of my k5. Your best bet is to hold the outer rocker up against the existing sheetmetal, draw lines to cut out the old metal, and weld it in place before doing any worrying about the inner. The cab mount is a bigger variable if you need to replace ( I needed new mounts on both sides). With the outer rocker welded in place, you just make the inner pieces fit as well as you can while making sure you can weld the inner and outer rocker together in a neat seam where they should meet at the very bottom edge. The only part you'll be able to see standing next to the truck is the outer rocker, everything else is hidden so worry about getting the outer mounted first!