Our trucks use 8 lugs on 6.5" bolt circle.
If your truck had 18" originally and you'd like to go back to that you'll be looking for old/used wheels.
For a modern equivalent tubeless wheel and tire look for 19.5" wheels and 8R19.5 tires are a good option. This is what I have on my 1 ton.
You may also fit some 16" wheels, check clearance on rear brake drums. Depending on the tire size they may or may not be as tall as you want.
