As others have said, gussets might be a good idea. Being that it's steel you would probably only need a small right triangle in the extreme corners of the stand to solve any issues that may arise. I just know that since I live in California, I'd be taking that precaution in earthquake territory.
As for wanting to put in a sump under the stand. Have you considered leaving yourself an access door at one of the ends? If it is a 6ft x 2ft foot print, you've got quite a bit of room for putting a pair of doors on the front and having a door on the left or right side. Then you can slide your sump in from the side(while empty) and still have your center brace if you choose to have it.
I just built a wood stand for my 120 that is going to house a 55 gallon aquarium for the sump underneath it. No way to get it in through the front due to the center brace and the two doors. Solution was to put a door at the end of the stand so I can slide the 55 gallon in from the left side. It also gives me one additional port of access to get to fittings on the sump or to have room for storage for supplies.