I just bough an old tank probable about as old as me 25 years old. The rim is pretty much shot in terms of cracked and i can lift the entire thing off in one piece. I'm currently in the process of resealing it. The bottom silicone is still in great shape but I have removed all the vertical corner silicon. Before removing the silicone is held every drop of water for 48 hrs.
Here is what it looks like.
I bought it as a 39 gallon tank.
The center brace doesn't seem to bare anything as it has tons of flex if moved. I was thinking if needed (on safe side) silicon a 3" wide 1/4" acrylic support piece in its place just glass to acrylic.