Undertank. Reason being that an overhead will drastically increase the ambient air temps. Also, you may want to consider going to get a thermometer if nothing else, if for no other reason so as to monitor your temps so that you can determine what you need to play with in order to meet proper temps. Consider a large kitty pan full of water as well so that if your temps go beyond comfortable and reach life threatening levels your
BP can escape to the water. Water is less prone to drastic changes in temps than air so it may save your snakes life if you happen to be at work and the air temps in the cage reach in the triple digits.
Always remember this for future reference: Less than adequate heat may leave a snake prone to illness or improper digestion, but hot temps will kill one.
Entropy,
ALex
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"We are destined to become the most well informed society to ever die from our own ignorance."
Edward Abbey