Well, I think I'd suggest a radiant heat panel. These tend to give off heat that spans a broader area. It heats the air so it'll heat the whole unit to a certain extent. There will be a warm side directly under the RHP and a thermal gradient as you move over to the cool side. With the proper sized RHP, the cool side should still be heated enough to remain within the proper temperature zone. With a vivarium of the size you're building, it's going to take a high wattage RHP. I wouldn't venture to guess on that - I'd discuss it with an expert. I like the RHPs from Reptile Basics and Rich can definitely help you determine what size you'd need.
If you need a warmer spot near the ground since the vivarium is so tall, perhaps you can combine the RHP with a UTH on the warm side. You would of course need a second thermostat or a dual-zone thermostat to operate both heating elements though....
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