Why?

If you put an offset wheel on (like duals that come from the factory) the
inside wheel won't fit under the bed.  For that to work, you need the axle
to be about 4" longer on each side.  Look at a truck with dual wheels and
imagine putting that on your truck. You'll see what I mean.

You don't have to do it that way, though. You could look for a dual wheel
adapter kit and use the kind of wheels you have now.  It's hard to explain
what that is - it's basically a spacer that goes between the wheels and some
hardware that holds both wheels on.

Those are the 2 possibilities.   Factory style, or using an aftermarket
adapter.

I second the "why?".

steve