That is too low of a temperature for your hot spot.
I use the SAME exact cage dimensions and my room is the SAME exact temperature, and I use a 65 watt Pro Products heat panel....which keeps a 78 (ambient)-90 (hotspot) gradient. This is my only heat source, I do not run an under tank heater as the heat panel provides BOTH your ambient temperatures and hot spot basking. Simply hang your thermostat probe on the cool end of the cage and set it to 78, this keeps your hot spot at 88-90.