Alright, here's what I would do given the situation, mind you this is not a quick fix but a really long drawn out one.
1) clean out some room in the garage for a while put down a sheet of plywood, basically a flat area you can work on this tank.
2) I'm assuming it's rectangular in which case you're good to go.
3) That overflow is most likely just siliconed right into place, so remove all that silicone and the overflow, put it asside (if it's glass wrap it up in bubble wrap or something to keep it from breaking)
3b) I'd put some sort of corner bracing near the bottom on each corner, because when you remove the bottom you're removing a lot of lateral strength.
4) Remove the silicone near the bottom where the piece is attached, glass should slide free with nothing holding onto it.
5) Get a new pane of glass (or hell I'm betting you can do a PVC panel like the AGE(??) tanks.) put that on the bottom and slide the rest of the tank on top of it (this why I suggest plywood on the floor, a tank this large is gonna be a heavy SOB and moving it is going to be a pain! But use wesson veggie oil or something to make it slide smoothly, don't need to chip the sides!) (make sure there's cushioning under!!!!)
6) Brace up all the sides of the tank so that they won't fall, and remove the rest of the silicone (you can NOT simply apply over the old stuff and expect a nice leak free seal)
7) Tape up along all the edges (a tank that size luckily you can stand inside and do this) to make a nice clean look
8) Gets some black 995 and reseal the whole damn thing, before it dries up toss the overflow back in and seal that up.
9) THen get it on a table so you can work under. And drill an overflow hole (be careful you don't want to go through all this crap over again!) You could also drill that hole before moving the tank.
10) If you manage to get this done, kiss the ground and wait a LONG time for the 995 to fully cure.
Again this is the LONG solution, it is not a weekend job. The size of this tank alone is going to make this a pain in the butt DIY repair job. If it were regular glass, I'd say just scrap it, but it's a nice tank, so ... if you don't feel confident you can do any of this, don't do it.
Of course my first thing would be to try to find a glass tank manufacturer and ask about a fix on this. Unfortunately I don't know who makes tanks in this area in glass.