Alright I think I understand I'm gonna take a crack at it, but if there is no high-poly model, how are the edges so smooth on his wall, just beveled edges?
Modify > Unitize, Select the inside edge ring, deselect the edge where you want your seam to be. At this point it should look like this: Then just click on stitch together.
Not sure if this will help you out and you may know it already; you can constrain your verts to edges by holding C and middle mouse drag on the desired edge.
it bevels it automatically, just divide it to see, the edges stay sharp because it is putting extra edges at the border areas. do a global smooth for a more pronounced bevel effect.
I suggest that outer edges' vertexes of every module fit together so that they can be nicely welded. Here's an image showing edges that don't match each other:
nice bake, the only things that really jump out are the hard edges on the sides of the tires and the edge of the rope on the front bumper could use a few more poly's for the silhouette
I like your style. But, the bump looks to centered and theres no cracking around the edges of the stones. I would break up the edges a bit. Awesome work though.