I have been looking for the same type of thing except I need a bridge to hold a 7,500 pound tractor. One problem I ran into is that people don't want to recomend anything beyond a foot bridge due to liability concerns. Have you considered a pipe culvert? Installed correctly they can support tens of thousands of pounds.
www.tractorbynet.com is a website similar to this but for tractor owners. There have been a lot of posts there about bridges/culverts/creek crossings.
