diy is fun, but will probably not be that cost effective for the sizes you are talking about. 55-75 gallon tanks are pretty cheap tho and you should be able to find one pretty cheap used on craigslist for roughly 100 bucks.
In my opinion, 2 GT's would be much happier in a 6ft tank, like a 125, and you could add a couple other fish. If you can't upgrade I might just think of starting over. It says ur from CNY and there are a couple local fish stores that will buy your fish from you so you can start over, I know one in Jamesville, NY right outside syracuse that will at least give you some store credit. There are tons of smaller fish that will be much happier in a small tank.