If you run singles on a dual wheel rear axle they should be centered with nearly no offset in order to load the wheel bearings as designed.
Might look odd or weird with wide singles on the back of an old work truck, and then you have different wheels front and rear.
It's up to you what you want to do and how you'll be using the truck, what makes sense with your needs in mind?
I personally would run 8R19.5 tires on appropriate dual wheel rims with either axle. Or swap to a 14 bolt and could run 16" if you choose. Advantages to a 14 bolt rear are gear ratios, brake parts, and wheel options. If you stick with the original axle you're stuck with some pretty low gear options, finding harder and harder to get brake parts and the stock 18" wheels/tires or find 19.5 wheels.
