OK, that explains the "extra braces". Oceanic only built those bigger tanks with a center glass brace. More like a shelf than a brace, but whatever works, I guess.
I think you have some choices.
Just replace the center brace in it's designed dimensions and remove the braces installed on the ends.
Replace the centerbrace and install chevrons in the corners.
Replace all three braces at 2/3 their original dimensions.
Eurobrace the tank perimeter and install a much small centerbrace - 6 inches.
If you can find a glass shop that will work with you, you already have a big chunk of what you would need to Eurobrace it. To be clear, lots of shops won't work with old glass or 1/2" glass. As I said earlier, this is probably the biggest hurdle folks face with project like this.
BTW I love your signature, my roommate is starting to see that the tanks are making us some money so she has basically quit stopping me unless we really don't have the money to spend lol.
The whole issue has "evolved" over the years. It's a rare occasion that she even steps foot in the fish room anymore. So when I get new fish, plants, supplies, tools, etc. it gets hidden in there until I can get it unpacked and in a tank or in the workshop. I almost always have stuff shipped to the office and I just sneak in through the basement with a quick drop-off in the fish room.
I couldn't hide the new band saw in there though.