The converstion from a swing axle to IRS can be done relatively easy if you have the jigs and a few fab skills. The reduction boxes that you talked about are off the old style buses that were also a swing axle. It was basically the same transmission as the bug but had reduction gears mounted out at the wheels that lowered the gear ratios. They were tough, but are hard to find and have some WIERD handling characteristics. The newer bus IRS transmissions from the 1968 and later are the prefered choice among most of the offroading crowd.