You're going to encounter a major problem with the inside dual wheels on the rear end. The brake drums are too big to fit a 16" wheel over them, especially a drop-center wheel for tubeless tires. The correct tubeless replacement for an 18" wheel is a 19.5", usually found on old 1-ton step-van bread trucks, etc. Check with a local bakery to see where they dispose of their retired delivery vans. They might even have replacement wheels available for a bargain. The 19.5's will be the same height as an 18" tube-type tire. Those tires can be somewhat pricey, too, but they eliminate the hassle with split-ring tube-type tires.
Jerry
