1. 41qt is just about perfect, maybe something a bit larger like a 45 for big females, but 41 is typically great.

2. Belly!! Much easier to control temps (thermo/rheostats). Besides, back heat dries out the enclosure, and with lights (of all colors--red blue, purple, you name it) you have to deal with photoperiods, which is a hassle, and then you have to worry about night temps. The only real back heat that doesn't give off light are the CHEs, and those REALLY dry out the enclosure. But I don't have anything against people who use back heat for their snakes. If you can keep temps and humidity stable with back heat, good for you. Back heat just doesn't make a whole lot of sense to me. But whatever works for you.