Reduce and Join massive meshes?

I have 72 meshes of LIDAR topographical data totaling 10 gb (.obj files)

The meshes are tiles such as those shown here http://imgur.com/rcaamcD (already used reduce 90% twice)

I need to reduce and join them into a single seamless surface with a polygon count I can actually manage (like 50 mb instead of 10 gb). So far, it seems like the reduce command erodes away the edges of the meshes so that there are gaps between the tiles.

I tried join and weld, but they didn't seem to make a difference (and if i did 'explode' after, I got my tiles back - not sure if thats important or not...)

Anybody have any tips or suggestions?