just did this earlier this year depends what you want to spend, Firestone and Goodyear both still make 16.5 tires about two hundred a pop, they can convert the size for you but most likely will have to ordered. be sure to ck tire revs as one size they recommend will make you run slower and the next size will closer to the 16 tires
i bought used 16 inch wheels from a truck salvage at $60 bucks each then bought 235-85-16 tires they are readily available just about anywhere and have a much higher weight carrying capacity and slowed my engine rpms down.
but check your clearances before going this way i think it will work ok as your rig looks a lot like mine i cked mine with my air ride all the down just in case it gave out.
put about 1600 miles on it so far with zero problems once i lowered the air pressure to 65 psi
hope this helps