I agree with Fly Guy. But rather than trying to find some extra hands... I'd cut another temporary brace and put it in off center enough that you can get the new sump in. Once it's in, the temp brace will make it easier to get the original back in. then remove the temp.
Like he said, big rubber mallet is your friend here.
Well, on second thought.. that's the same stand I have under my 120. I just took a look inside.
There is a cross brace that sits on top of that center brace. I had to take that cross brace off my stand to get my 40g Breeder turned sump into my stand, but I didn't have the display tank on yet. And it popped right out with my rubber mallet.
here's what I think I would do.
Start by cutting a new brace (red beam) that will fit between boards A and B. The original center beam (board C) has a rabbet on both ends, and mostly is just supporting board A on board B. So you don't want your temporary brace to be inside the stand, you want it sitting directly on B and directly under A. Make sure it's a good tight fit.
Next you're going to have to figure out how to get board C out. As I said, there is a cross brace between Board C and it's twin on the back of the stand. It it not in contact with the bottom of the tank (not on mine anyway) so you should be able to remove it, but be gentle about it... you're going to have to knock it up towards the bottom of your tank. might be wise to put some towels on it to dampen the blow if it hits your tank. Go slow here. Once it's free, now you can knock out board C. You put your temporary brace in already right?
Insert new sump.
While board C is out, I'd remove any nails sticking out of it, and also take a chisel to any large ridges of glue just to clean it up a little. put some fresh glue on the rabbets and put it back in place. you may need the mallet again for this. And if you have access to one, I'd take a brad nailer to it to secure board C again from the inside.
Getting that top brace back in now may be a bit of a challenge. Since it's not a weight bearing piece though, I don't think it'll pose too much trouble if it's not back in as good. Again, I'd try to clean out any nails sticking out of it and definitely glue it back in place. Better still would probably be to then get a couple small L brackets to screw it back in good and tight. Since we can't get to the top of it to nail it back in like it was originally.
