I use button head hex for that sort of fasten job. BTW I don't know how a dump bed fits on the frame but you might want to check out the assembly manual because to move the crossmember might create a minor problem installing the bed. I assume you will have to bolt some parts down back there and if the crossmember is in the way you may have to drill through and use longer bolts. That might be the least of the problems you will face.

~Jim