This from Mick McDouggal, FOI member:

   I didn't do this conversion, but the topic comes up often so since I
   just replaced my power steering gearbox, I thought I would go over
   how it was done.  My bus is a 42 clipper but the steering looks
   much like my later model 47.

   It seems incredibly simple to convert.  The only trick part is a
   mounting plate for the steering gearbox.  The plate is manufactured
   out of about quater inch steel.  its about 12 by 14 or so inches and
   has 3 holes for the mounting bolts and 1 large round hole for part
   of the steering gearbox itself. This plate has another plate welded
   to it perpendicular to the main plate and this mounts to a brace
   that runs from the outside frame to the inside isle frame.

   The steering box is a Roxx HF5400 series, the exact number for part
   comparision is a HF545001 and the number is found on the flat area
   around the output shaft.  I paid $500 delivered overnight for a
   rebuilt one from California through 6 states distributors.  This
   has a standard Pitman arm on it and it looks like they cut the
   original drag link in half and welded it to the link that connects
   to the pitman arm.  (Ed note: in most jurisdictions, any welded
   part of the steering system is a no-no.)

   The box is fed oil via 2 rubber lines that
   originally went all the way back to the motor underneath but I just
   re-routed them through the interior channel under the drivers side
   floorboard.  I looked at a 1990 Bluebird school bus with a cummins
   in it and it looked like the power steering gearbox from that could
   be modified the same way. So if you need to convert, you need.

   - Powersteering box. (Ross HF54 pictures available on internet)
   - Powersteering pump.
   - a mounting plate made.
   - a modified drag link to mate up to your bus.
   - 2 long power steering lines.

   From my research, I found used powersteering boxes for $100 and
   pumps for less.  The lines could be expensive, but if I had to do
   it from scratch, I would use long copper lines and just use shorter
   rubber lines and mate them up with standard copper line fittings.
   So you could reproduce this setup for under $500 or so and a handy
   guy with a welder might be able to do it for under $300.
   Just an FYI
   Mick