As far as I know Lee does not make a 150mm adapter, they make the SW150 system, which is designed to fit the Nikon 14-24. Lee has recently changed up the light shield for this setup and may now make an adapter that fits to 105mm, but I have not read about it.
Lee makes a 105mm standard ring for their 100 x 100mm filters, but that will not work either as the ring when mounted to the front of the lens will vignette, and the 100 x 100 won't quite fit the 105mm opening of your lens considerations.
I am looking at the 105mm Hitech ND solutions in the Firecrest lineup, but I don't use grads.
There are other brands out there like the company that makes the setup for the Sigma 14mm, again it's designed to fit over the tulip hood.
You would need the Lee 150mm x 170mm grads or 150 x 150 solids, and may need to look for a custom adapter being make like from a company such as SK Grimes, (in the US).
Depending on your back in use, CMOS or CCD, if you are using a CMOS back with your Phase One camera, you might be able to hand hold the Grad as long as your back is on a tripod.
Paul Caldwell