The m/b I'm using is 0.2" too wide to fit a 1250 there. (just a shade over 3.5" available in mine).
You could make the res the approximate size of a 5.25" drive if you have 1 slot free in the top cage. Then you could access it from the front
Or put the res at the top, between the front of the case and the radiator with a fill port going out the top of the case.