I've cut new glass and made my own tank. I do a lot of metal and acrylic fabrication for work but I barely trusted the 40 gallon I made. I also tried taking apart a 55 gallon tank to make a sliding front screen chameleon cage with a screen top and after scraping out every possible piece of accessible silicone, I broke the first panel. I then tried learning on a lost cause and tried chemically softening a panel and heating another. Out of five panels only two came out intact, but their coming out whole was at the sacrifice of the connecting panels.
That was many years ago and I think I am better skilled now but if it were me, I'd sell the 120 and put the $ toward what you are after. I believe you are thinking about cutting all four sides down to make a shorter tank. Glass cutting involves scoring a side of the glass, letting the score face up while hanging just over the edge of your work surface and sharply taping it with the other end of your glass cutter. The silicone seams in between the portions of the panel you want to remove will inhibit the ability to get that sharp tap which causes the glass to fracture.
If you go for it do a photo step by step and share your success or failure with us. Good luck.