A 100w bulb is only really suitable for a lizard. For a snake, a 25 or 40w is plenty.

For main heating, always go with the UTH or heat tape, since snakes gather heat from below more than above. Especially BPs. And always be sure you have a thermostat to control the heat source.
Human heating pads are NOT a good idea, since they are not designed to be on ALL the time. They're great for emergencies or to heat a holding tub, but not good for home enclosure heating. Look into getting an Ultratherm pad, Flexwatt heat tape, or a ZooMed UTH. Size should be enough to cover 1/3 the bottom of your enclosure.