Scott, clamp your finished billets down flat, and lay a straight edge along the side to line them up straight. Then lay out the Z-splice, and be very carefull when you cut them out, just leaving the lines. Draw your center line first, then measure 4 inches out on each billet and mark that, then connect the dots so to speak:) Just like the last picture above shows. Once cut out, if there are any problems, I use a rasp to get the cuts even closer. There may be tight spots keeping the two pieces from sliding together all the way. Get it snug together and hold it up to a light and take notice where the tight spots are, mark them and use the rasp to gently lower those spots. Don't worry, my first few Z-splices looked like Ray Charles did them:)
Good idea is to screw a 1 inch piece of aluminum angle to the edge of your work bench, makes clamping things down a snap.