I rebuilt a 75 gallon aquarium and added two new panes. Could it be to tall?

A friend gave me a 75 gallon pet store fish aquarium. You know the low front pane, high back pane, and divided in the center deal. And the sides were higher then the front and back. Okay I rebuilt the tank and removed the front pane, back pane, and the divider. I had two new pieces cut to fit the front and the back. About 52 X 26. So I asked a local pet shop (mostly fish) if there was anything wrong with it. He said I might be to tall and all the pressure will be at the top of the tank. And im adding a brace in the center for that. But is there any other way I can reinforce the tank so I could get a long use out of it. The tank is now about 120 to 130 gallons. The tank is 52L X 26T X 18W. The sides and bottom panes are 1/2 inch thick and the front and back are 3/8 inch thick. This tank is all plate gl. Just for some extra info I had the tank full for of water for about 72 hours str8. Didn't have any leaking or pressure cracks or anything. Again any information that could help me make this a long lasting tank would be great.