Let me explain some of my thinking to make it a bit more clear?
We know that an angle iron stand has some reasonably small iron involved and we can press down on the center of the top horizontal and make it bow down. Yet we know that it holds the tanks as designed. Since the center bows down so easy the weight has to be supported by the portion over the legs where we can't press it down. The center can't bow down because the sides won't bend.
Example A in the picture below? We know this works and feel fine with that.
Example B is often used by folks who want a quick, cheap and often temporary stand. Like college folks who move often? They build two stacks of concrete blocks, throw a couple 2X4 across for the tank to rest on and it works even though you can easily bend a 2X in the center of a 3-4 foot span. The weight is supported by the legs and the rest is just floating because the sides won't bend.
Example C? We don't like it! it looks really bad and unsafe. But where is the weight resting? The weight is still supported by the legs and the rest is just floating. As long as we don't go crazy and extend the ends to the point that it topples left/right, the tank is okay.
So when you ask about gussets and extending the ends, I feel they make us feel better and it looks "right" even though the weight is still going to be supported by the legs just the same as without the extention and gussets.
All this has to be done with some reasonable limits but for 6" either side, I see no problem.