Luckily I had a mate over and put him to work!
I first aligned it as far to the right as I could, without the tyre protruding the edge of the van - using the top tailgate lip to slide along (for those of you who haven't fitted one of these already).
Then, after drilling one pilot hole from the outside, I was pleasantly surprised to see I wouldn't have to cut away any of the inside door frame.
However, I could hear plenty of swearing from the inside, so it was pretty tight with the backing plates fitted in there too. Using the locking nuts provided made it much harder for him. Normal nuts with washers or flanged nuts would have been much easier.
And I used a step drill to finish off the six holes. It gives a much neater, rounder finish.
I also replaced their bolts for A2 stainless (but not the locking nuts!).