The 5831 has been obsolete as far as commercial use goes since the 70's and parts are not readily available. I have seen them go through salvage auctions for scrap iron price. You might call some of these people to see if they still have parts.
http://www.partsbyweller.com/truck/WTlocations.htmhttp://www.valtruck.com/contactus.phphttp://www.geigertruck.com/http://www.chalks.com/index2.htmlI have dealt with all of the above companies and they are reputable rebuilders.
As for your discussion of road rangers I don't fully understand your reasoning. If you are speaking of the 2 bolts on the back on top of the transmissions, some truck manufacturers put a single leaf spring there which was usually omitted once the end bushings wore out or the spring broke. I have logged many million miles on tractors with Fuller transmissions without rear support. Cabover Internationals of the 70's were the last to use the spring leaf in the rear. As far as saying a RoadRanger has too much torque for anything over 1 ton makes no sense to me at all. The old 600 series Fullers and the current production FSO series transmissions are specifically built for 2 to 3 ton trucks.
