I have bought a second hand ARB roof rack off a Toyota Hilux- hard to knock back for the price, but it has different mounts, all of which didn't come with the rack anyhow (vehicle was written off with the mounts fitted, but not the rack) and I want to fit it to the rear of the Patrol.
Also, are the mounts for the Nissan racks fully removeable, in other words, normal gutter mounts?
The reason for all this is, the rack is a square peg, and the GU mounts are a round hole. ARB says it cant be done. This rack wont go onto GU mounts. I reckon it can be done, with a little tweaking.
This is what I bought

I am after the distance front to back, centre to centre, of the 2 flat mount plates, shown on the far side of this pic

And then the width, inner edge to inner edge, of the flat plates on the mounts that bolt to the underside of this adjustable plate on the rack.

I currently plan on buying a set of mounts for the Nissan 3800030 rack, and if necessary, drill up a set of adaptor plates to suit mounts and rack, out of 1/4 inch plate steel.
The worst case scenario is I mount the ARB rack on a set of Rhino bars I own. But I would prefer not to do that.
I guess ARB makes these things up in different sizes, not only to suit different width vehicles, but also to thwart people like me doing what I am trying to do, so I have to go out and pay full price for a new rack and mounts.