You don't need to heat the tank at all if your room temperature is nearly a constant 75F. What you need is a basking light that brings up one or more sections of the cage to the proper basking temp.
I would start with a 100-125 wall outdoor flood style bulb. Set it up and check the temps of the basking spot in about half hour. If it's too high, either lower the baskign spot or raise the light. If it is significantly too high, then you will need to go to a lower wattage. Y ou can also just buy a 75, 100 and 125 watt bulbs then bring them home and fiddle till you have the right range of basking temp. It is possible that you may have to adjust later, especially in the spring/fall when temperatures fluctuate a bit more.
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PHLdyPayne