there's no way around the hit-and-miss placement of fencing - or any models for that matter - that you want to match end to end.
you can, however, cut out a lot of the misery by placing your first two, join them up, copy both together and paste a new pair, match the new pair with the first, copy all four, paste a new four, etc... doesn't take too long to get 16, 32, 64, 128 bits of perfectly aligned fencing...
you can then save those huge lines of fencing in a test mission, and when you want to replicate those lines, just load up the mission, select however many fences you need, copy, load up the new mission map and paste. voila, instant fencing