If the room is 68 degrees then a UTH probably won't get the ambient temps you want andwill probably have to use a small ceramic on top of the uth.
I'd use the rheostat for the ceramic and pick up a cheap thermostat to control the uth.
If you want to save some money buy flexwatt from reptilebasics or the beanfarm and wire it to a cord and plug that into a thermostat instead of a store bought prebuilt heat pad.

11x12 flexwatt = $3.60 a foot VS $20+ for a prebuilt zoo-med or Zilla heat pad or other store bought.
With an 11X12 you can cut it down to cover 1/2 of the bottom of the tank which will help some with your ambient temps and still give the snake room to cool off on the cool end.