Hey Shazz! Welcome back. Glad to see we have the same inspiration! 😀
On mine the 3x pup rings just fit, but it's custom order as you saw so will be worth checking how much clearance there is between the 2x pups on std kit.
I am using a P90 in a humbucker format for my middle pickup, a tonerider rebel90. It looks great with the row of screws in the middle!