It's not that big tanks are 'evil'... though there are many here that think tanks of any size are a write off. No, it's that a 55gal is way too TALL to properly maintain the heat the way a snake needs it. I have my biggest girl in a 40gal and had to do some creative insulating to get the temps stable (which they are) because of the height of the tank.

Make sure you have UTH or flexwatt heat first, and if that proves to not be enough THEN add lamps. Snakes get their heat from heated ground, so lamps alone don't quite cut it for them.