The "moving bottom" is called an apron.
Examine the bottom sides of the chain links on the apron carefully for excessive wear. A common failure with manure spreaders is that the apron chain breaks. Of course, it always happens with a full load, so you end up pitching the load off by hand. Been there, done that, multiple times. And yes, if the manure spreader stands loaded for a long time, the wooden floor will rot out.
I think you're going to be disappointed with how unevenly a manure spreader will spread dirt.
I was trying to think of the perfect machine to spread topsoil. There are fertilizer spreaders that have a tapered bed (for flow) with a narrow apron down the middle. The apron delivers to a horizontal whirling broadcast wheel at the back. If you've been behind a salt truck on the highway, you'll get the picture. But, it's probably going to spread too thinly, requiring many passes to get thick layer.
Farmers rent out these granular fertilizer spreaders in the spring. Not sure a rental outfit would appreciate you filling their machine with dirt, but it is the off-season now...Check at a local Ag Co-op. Usually, they will deliver the fertilizer spreader to the property...filled with fertilizer, of course.
Maybe ask a large landscaping outfit how they do it. Or, ask at the place that sells the soil.
I think I'd deliver the soil using a FEL or skid loader, tipping the bucket just so and backing up at the same time to spread it. Then use a drag harrow to distribute it evenly.
-Paul
