Scott, like I said, your spacers are going to be a custom fit. Your best bet is to simply put the axle in, center the wheel, then get a dial caliper in there and get a measurement. Be sure and take into account the fact that your wheel bearings will be recessed about 1/4". You need to measure that and add it to your spacers. Remember also that the axle itself has a shoulder on it, and that will act as your outside spacer on the left (rotor) side. When the axle is installed it will be recessed about 1/4" into the left rocker. Also be sure to use red locktite and crank the shit out of those clevises, I used a piece of 1x oak. Make sure they are mounted in the trees, parallel to each other and square. Any machine shop can make you spacers, but even better is to find a small builder in your area. Cut them from polished stainless if possible. Aluminum is ok, but gets dull and it's hard to get in there to polish it. With the springer you will see them.
Staffy when you are ready to cut your tubes call me. I didn't have to cut my main tubes, but the spring tubes will have to be trimmed for sure. Just level your frame, put the uncut tubes in the trees with the clevises attached, and roll the wheel in there. Cut the main tubes if necessary, and then fit the spring tubes according to where you want the spring pack to sit. I used a 10" metal cutting wheel in a chop saw to do mine. Clean the burrs off the inside on the clamps with 400 grit paper, and use wedges. It's very easy to scratch the tubes, especially if you get the black powdercoat.