Once the 55gal tank was emptied, the question became how to separate the glass panels. A razor blade and a lot of tooth floss are a good starting point. The first seam is the hardest, once it is unglued all the other seams are comparatively easy to separate.
After half an hour with the razor blade, my patience ran out and I grabbed a hammer. Since I didn't need the side panels, smashing them carefully sped up the process immensely. The bottom panel of this 55gal tank was tempered, and to compare some I took the hammer to it as well. I found that tempered glass is much, much harder to shatter, and once it broke, it broke into hundreds of little pieces as expected. They are still very sharp... ouch.
As a side note, it might seem a bit weird to cut up a nice 55gal tank, until you realize how expensive glass panels can be if you buy one or two. With applied economies of scale, buying a complete 55gal tank is cheaper than buying the separate glass panels!
The floor in my garage is angled towards the street, and curved towards the center. Not a good base for a large tank. Therefore, I decided to build a concrete base. Soon I realized that this wasn't as easy as it seems, but after a couple of layers it was all built up fairly straight and level.
I am planning to add another layer of fresh thinset just when lowering the plywood construction onto the base, to make sure it is well supported and level.
To keep energy costs reasonable, I removed the drywall and filled the space with insulation. This should help to keep the tank temperatures cozy during icy winter nights. The plywood should insulate a little bit, and I will cut a fitting piece of styrofoam insulation to cover the front glass. As you can see, I also painted the garage floor with epoxy paint. Very easy to do, and makes a big difference in appearance.