If it is 73-75F in the tank during the day (and also 68 in the tank at night), it makes me wonder what the room temperature is... aren't YOU freezing?? I use a UTH and 50W red bulb for my BP (day AND night!), and it's a steady 80F in his tank (warm side at 90F), while it's 74F for me in the apartment. For the winter, can you not turn your heat up? Or like Ricky said, a space heater would probably work wonders. You can find those cheap on Craig's List or Kijiji (if you're in Canada). Or even at a big-box store.

I'd also suggest instead of having two different wattage bulbs, get a higher wattage night bulb - as well as keeping the UTH on. You might have to mess around a bit to find your "perfect" temperatures.
Perhaps take a gander at the care sheets on this forum as well, if you haven't already very helpful!

Good luck!