Up up and away.

I had obtained some scaffolding boards and concrete blocks in order to construct a safe work platform. The platform made the lifting and inserting of the gravel boards much more manageable.
As each gravel board went in I placed a thick bead of frame sealant along edge of the board. This was done to provide a water and wind proof seal between the boards. We also inserted more packers to wedge the gravel boards firm. Rather than completing an entire section all at once we worked around and built it up gradually. This ensured we didn't push out any if the posts more than any another. One section was a bit tight so we carefully spread the gap with a ratchet strap and after the first board was in it all fitted nicely. Progress was quick and soon all the boards were in place. Next we marked around the post tops with a black marker and cut them all off level with the final gravel boards. I used a angle grinder with a good quality diamond disk. I made several cuts in each working towards the middle. The end result was good clean and level. I think having a good disk is vitally important.