Is it a solid base of large rock? If so, some smaller rock (with fines) may work well to smooth off the drive. Usually 1" minus with fines will blend in, fill the voids, and set up well.
About the best drive base material (in my opinion) is crushed concrete, or crushed concrete mixed with crushed asphalt. (con-bit) Straight crushed asphalt also works, but if you're planning on paving when the heavy truck traffic is done, I don't recommend paving over it. It bonds funny, and you could end up with an uneven surface in a few years.
To do a 500' drive, 12' wide, 4" thick; you need 75 cubic yards.