The innovative marine internal stand dimensions are about 32":
If you elevate the sump a little and place the sump on extruded aluminum frame on the bottom of the stand, you could might fit the 36, but it would be very tight if it did fit. The IM stated measurements are slightly conservative.
If it was me, i would limit my sump length to about 30 to 32 inches. Not for removal purposes, but because you want some room around the sump for cords or the like. Here is my very tightly packed sump on IM 50 gallon, six inches less long than your IM 75. This took a lot of planning and pre-measuring of gear when designing; i have never seen anyone else try to cram so much crap in a sump (especially since there is a 50watt UV on the back). The sump is a fiji 24, stated width of internal of stand is 26 inches. Although it fits, i have no room for an ATO but i do have room around the sides and front for things (e.g. running dosing lines!)
Also realize that the down side to a big as possible sump, is that you have much more cramped working area, which can make maintenance a chore; this is the reason people build fish rooms.
You can certainly fit a 20 inch sump under that tank and if set up right, would be enough volume, while giving you more space to work.... It's a personal preference.
If it was me, i would plan out your sump -- and what other gear you may want -- as well as design your cabinet on a piece of paper using gear's dimensions stated on line before buying gear. That way you can pick gear knowing if it all will fit or not.
P.S. i run a klir and love it...