So... I'm still a novice at this, but I've put in a lot of time reading and researching. I know from what I've read, not from experiences with different setups myself.
So, given your situation, Here is what I would do if I found myself in your shoes... (I'd consider a new tank, but I know you don't want to do that.... but really, it would make things easier... but I digress)
... for a tank of that size, I'm guessing it's fairly long and high... I would consider getting two CHE (ceramic heat emitter) bulbs, 100w (if not bigger) and put them on lamp dimmer switches. You could put one over the cool side of the tank, and another one in the center of the tank. One on the cool side may be all you need, so you could start with that and see where it gets you, but know, you may need more, depending on the height of your tank. Then you can use the dimmer switch to control how much heat the che puts out. It will likely need to be cranked up all the way. This MAY raise your ambient temps, but it will also cut your humidity way down. Trust me on that... my che cut my humidity almost in half in a small plastic enclosure.