So I can see that their standard Hipshot A prep for 5 string is for the 17.5mm (.687") and you want 18mm (.708"). This would only (maybe) be a problem if you wanted to string through the body. I bet the mounting screw spacing is the same across the board, you may check with Hipshot on that, they are very accessible and responsive usually.
You could also ask Warmoth if they could adjust the spacing for the string through but they tend to be less flexible on this kind of thing.
Alternately, you could just get the 17.5mm and space it out as much as possible, you may or may not get to 18mm. Doesn't sound like a lot but it's a total difference of 2.5mm. You'll probably have to just about pin the B & G to the outside and space the others evenly... this won't give you much adjustment if needed to match the pole pieces of the pickups.