There is not that much to narrowing a housing and I have done it poor boy style with a friends home made kit. All you have to do it make or have made a set of solid spacers to replace the bearings in the center section and in the axle tubes, and by a quality piece of straight 1.5 inch bar stock. The inserts will need to be made so that they are a slip fit with around .003 or so clearance on the shaft. The axle tube end spacers should have the same clearance on the outside where they slide in the tube ends. The spacers for the center section should be the same size od as the bearing race so they will be held solid with the carrier caps.
Now that you have the parts, you simply cut the tubes to the desired length and use your new parts to align and hold the tube ends as you weld them in place. I will add that any bracing must be added to the housing before inserting the tube ends. My housing pulled back .125 inch when I added the back brace. You will notice that the tube ends do not line up perfectly because of this. Dont sweat it. Just grind a nice taper so you get good penetration when you put it all back together.