I was messing around with this also… same thing the seams did not match up 100% and fixing then using the ue4 landscaping tools is a huge pain.
I was thing about other games with large open worlds … wow came to mind. If you go make and look at the original zones before you could fly around… the edges were mostly impassible areas Like mountains or oceans. with small entrance and exits.
Now with a huge budget they changed most of that to more open areas and with flight.
My suggestion with a smaller indie or if your doing it solo would be to break it up in smaller chunks and only fix seems on small entrances to other areas.
The biggest area I have been able to work smoothly with world machine was a 48.774 x 48.774 and used the 8129 resolution then scaled it in ue4 by 600% and 1000% for the height… I set the height in wm to 5120
the skybox and lights would get bad much larger then that. had to scale them up also