If there's really on Chromis in there, and maybe a little bit of LR
that I can see in the pic, consider getting some tubs to collect the water in and draining it. Getting rid of that water weight you can remove the center brace, install the sump, then put the brace back in.
It's really risky doing it any other way if there isn't a way to manipulate the sump into fitting through the door.
Can't see the construction inside the stand from the pics - is it possible to remove the side panel while still leaving the supports in place, or is the side panel part of the supprt?