Best bet would be to find some 3800 dual wheel axles, front and rear. I don't recall how the front hub and or adapter works, parts manual should help figure it out.
18" tires are expensive now. if you did not use the truck much could probably find some old half used ones to last a while longer. New ones can be bought from a number of specialty tire suppliers. The 18" wheels are not odd or hard to find at all, most all AD 1 ton dual wheel trucks have them.
19.5" wheels and tires would be the way to go if you planned to use the truck a lot.
Grigg