Thanks for the comments guys.
Lynn. The small brass bolt come from Walthers model railroad parts. 0-80
http://www.walthers.com/exec/search?quick=hex+screws
I've also used American model engineering supply, and they have brass and steel.
http://www.americanmodeleng.com/id14.html
Jordy Great idea and I thought about that, but would make it more complicated to install steering axles.
Joe and Jason. I think I'll go with your suggestion tandem front and back, What I think I'll do is make the box a 20 footer but keep the full length trailer. I'll put the box centered on the trailer like a transfer, but have it dump at the back. Should be able to build a screw drive that will move it to the back and then dump. here's an idea, What do you think.
Sorry for the poor image. Hard to see with white background.
Rob