Rear travel is a little more complicated to figure out than the front. It's because of the angle of the shock. With an angle on it, the shock will move less than the axle. For example, if your shock is at a 45* angle to the travel of the axle (factory is less, it is just easy to make a point with 45*), and you compress the axle 5 inches, the shock will only move 3 inches (I rounded). If the shock is strait up and down (like the front) then it is a 1:1 shock travel VS. axle travel ratio. The angle of the rear shock complicates things (plus the fact that the angle changes as it cycles. I have 7" between my axle and bumpstop and 5 inches of shaft showing, but when I cycle the rear suspension to the bumpstop, the shock isn't bottomed out yet.
You can try to do the math and figure out the rear travel and where your shock needs to be mounted for the most travel, but the only way to really get it right is to take all the springs off except the main leaf and then you can bottom out the back and droop it to see where your shock should be.
Good Luck
Marcus
Those mounts look really good! I'm trying to do something similar and I appreciate the pictures tons!