I'm having temp problems and need advice

[Bogertophis]

I have a heater on in the room set at 75°.

Okay that's a pretty warm room then. I would expect your UTH should be big enough but you have to test that out- to keep it regulated in safe range for a snake living in the terrarium (as far as surfaces they may contact, as previously discussed). If you need more warmth, an overhead bulb or CHE should be enough.