Why chance things? If it were me I would keep the stand as is and go with the smaller sump. Figure out how to make the smaller space work. If you modify the stand to get the larger sum in, think about what you'll have to go through to remove it if needed down the line.
As far as the brace goes it's most likely needed, mean it's structural and vital. Even if you were to redesign it and make it removable, you would still have to pretty much drain the tank if you needed to remove it for any type of maintenance or to remove that sump.
Again, if it was me, I'd leave it in place and find a workaround for whatever you needed to do in the sump, not sure what your plans were for a larger some, but I'm sure there's a workaround with the smaller sump. Also, there's really not enough information provided to give sound advice on modifying that stand. How big is the tank, what's the stand made out of as a solid wood, partical board or plywood, are the corners reinforced, the back looks like it's already been compromised by having several holes cut in it… My 2 cents.