Yeah huge mistake... I did something similar. Way back on one one of my 55gallon freshwater tanks, I built a sump out of a 29 gallon tank, thinking that it would fit. Well I got it all done, went to put it under the stand, and to my amazement, I realized it was too big to fit.
So I have a sump that doesn't work, never did try to fix it. What I ended up doing it taking apart one of my old 40 gallon breeders (that had cracked) when I was using it for my Chinese Water Dragon.
I'm going to have the Local Hardware store cut the glass for me, (I have the tools, but I want it to be perfectly straight) because they have an awesome slide type tool for cutting glass and plastic.
My measurements will end up being 24 long, 14 tall, 10 wide. It ends up being 14 gallons full, even if I don't fill it all the way, it should be enough room. If you have any old tanks you don't use, that's always an option. Or redesign your stand.
Sorry for your bad luck, but just know, it happens to other people too