My two silvers are currently in a 6 foot tank. They are 26 to 27" long. A silver aro needs a longer tank, once it is 30 plus inches the 6 footer will become quite cramped.
The 8x3x3' would be a great tank for your silver. You will find however that the 3' height will add a lot of cost to the price of the tank. I will be paying almost $700 more just to get a 8'x3' with 30" instead of 24" of height.