rear shock stays where its at. your basically converting your rails from 136 to 141. all the hole sizes are on the rails. unbolt the front arm and lower shock mount, as well as the crosshaft and thats the hole size you need. use center to center for the measures, measure carefully and precisely as you dont want to rack the skid.
after some further research, if you complete the rail mod, the skid will match holes in the tunnel and bolt up, going from memory.
one thing to watch for before drilling any holes is where the new holes will come close to the existing holes. precautions must be taken here. if at all possible, lay out the new holes with a sharpie and before any drilling, clamp the arm and shock mounts to the rails to get an idea of what your accomplishing. then whichever mount it was that had to be lifted slightly to clear an existing hole will be evident. you want to be sure and have plenty of material on the rail when drilling new holes. you should see what i am talking about. wish i had one here to compare.
couple of drilling tips: use a center punch first and start with a smaller bit, around 3/16 or 1/4 for the pilot hole, then work your way up to the final size. 25/64 comes to mind for final hole size. shock mount may have 2 sizes of bolts.