I like tall tanks too. They are so much more visually appealing. But you are going to pay a premium for an 8'x5' sheet since standard sheets of acrylic are 8'x4'.
I like the idea of using thick acrylic, but a single 8x4 sheet at 2.5" thick is going to weigh around 500lbs. If you are talking about going all acrylic construction, the tank is going to weigh over a ton. You'll need some skill and a forklift to build a tank that size.
I would also ask Acme Plastics why under the "Applications" section, aquariums are not listed. From what I have heard some pro acrylic builders say, there are some cheap cell cast acrylic manufacturers that you should never use their product to build aquariums with. With how cheap those sheets are, I'm guessing they are not aquarium grade acrylic. If you use one sheet just for a viewing window it might be ok, but from what I've heard it's the seams on the all acrylic builds that can be an issue when using cheap cell cast.
Looking forward to following your build thread.