I don't think that will fit exactly as the height of the PI on the header might not be exactly right. One you have it built test first.
I used 20mm M2.5 standoffs for my PI but I originally cut the holes for 18mm. I need to shorten the standoffs ever so slightly to get a better fit.
You need holes between the NIMs and maybe to the left side to facilitate air flow past them. Holes in the case under the fan. With the supplied rubber feet keeping it off the deck the air is sucked in through those holes, blown onto the PI and then escapes past the NIMs. Works well in keeping it all cool.
Mike