Flexwatt can run 120 degrees all day long and not hurt it at all.
What it will hurt is your snake.

You will find that flexwatt doesn't do a lot for ambient temperature especially if the top is open an lets heat out easily.

I'm assuming your controlling the flexwatt with a thermostat to control the temperature?
78 is fine for the cool end, was the whole enclosure 78?

Got pics?