http://www.redtailboas.com/general_c...eral_care.html
There's some information I found helpful. I don't think a single CHE will be enough. I would use a UTH to get the warm side up to par. I keep mine at 90-92 for the hot side and 77-82 on the cool side, depending on the temps in my apartment. If you're using a screen top and a CHE you're going to have issues with the humidity, so be prepared to deal with that by choosing a substrate you can spray or having a humidity-raising device.
I'm not sure about the enclosure size, I got my boa as a juvenile, so I'm not sure if 40 gallon will be too much space or not.