If the room you house the snake in cannot be kept in the high 70's you should either as said above add a second UTH and thermostat or buy an oil filled space heater to bring the ambient temps up in that area. Having your cool side dip to 73 is very dangerous for your snake. You need to take care of that ASAP.

Keeping humidity up in a glass tank is tough as it is and when you have a lamp sucking out all of your humidity it's that much harder. Try the damp towel technique and see how that holds up. I have a PVC vision cage which is built for herps and I still have to mist twice a day during the winter months to keep things right. During the summer it usually stays between 50-60%

You also may want to look into a tub setup with the best interest of your snake in mind.