leeherps,
Wow! If you can pull it off you could have a zoo-quality exhibit when you're done. How many GTPs are you going to house in the exhibit? At the zoo I work at we keep a breeding pair in an exhibit that measures roughly 30"w x 30"d x 5'h. We simply have large branches placed in the exhibit with a pothos plant draped over a very large lava rock and a diffenbachia to one side to fill up the space. Since they're largely perching snakes they don't tend to move much. Still it'd be neat to have a really huge exhibit and see how much the move and what spots they'd seek out on a week-to-week basis. Anyway, onto your question. Your major problem besides finding large enough plants is going to be providing enough light. I'd recommend using compact fluorescents combined with something like a large 4' fluorescent fixture that has 4 bulbs in it. I'd use full spectrum bulbs in the 4' fixture. Mind you I don't mean UV bulbs but simply bulbs such as the solar bulbs put out by GE that produce a full *color* spectrum that mimics natural sunlight. Unfortunately it seems that most compact fluorescents are not sold in a full spectrum type yet and as such they tend to put out rays that accentuate yellows and reds giving an artificial look to what they light up. That being said they put out a tremendous amount of light for their small wattage and plants to do very well under them. Now, as for the plants you'll be using, you might try some ficus species like Fiddle leaf fig (Ficus lyrata), or perhaps Rubber Plant (Ficus elastica). You could also look at Hybiscus, some of the large Philodendron species, and even palms although I'd caution against palms as most do poorly in the house and would likely do worse in a large terrarium environment. The main thing is going to be finding large branches that you can position to provide perching spots and then position the plants so that they provide the cover over and around the branches. You're essentially giving the snakes large sturdy branches to perch on and the cover they seek is being provided by the leaves of plants you've placed near to the branches. You can even get creative and attach small hanging basket plants such as Pothos, Philodendron, and various ferns to the crooks of large branches, draping the leaves over the branches and concealing the pots with moss. I hope this gives you some ideas. Feel free to email me if you have any other questions.
Matt Campbell