Heating pads for reptiles are generally referred to as "UTH" (under tank heating) & they never go in any enclosure- they go underneath. Safety depends on correct installation (they need a small air gap to prevent overheating; also you don't want anything resting on the power cords). UTH will not usually be enough heat, especially for a large enclosure, unless your room/home is kept very warm (like 77*) year-round, because BPs need about 90* at one end of their home, 24/7. (or just under 90*) The larger the enclosure, the harder they are to heat adequately- especially for snakes like BPs.
It's generally impossible for any of us to tell you exactly what you'll need to heat a new enclosure. Why? Because it depends on how warm or cold your room (or house) is allowed to get- and it depends on what kind of heat sources you're using too. Insulation on a tank can only do so much. A larger tank or PVC enclosure will likely need more heat- likely 2 heat sources.
Many here use a combination of a UTH (aka "heat tape") and overhead heat source- usually an RHP (radiant heat panel). You can look at various options here https://www.reptilebasics.com/heating & other sources/brands, that's just an example of one source for reptile products. Here's another: https://beanfarm.com/collections/hea...EaAu6MEALw_wcB
No matter what you use, please (!) set it up for at least a week to test out all your equipment BEFORE you put in a living pet. It can take a while for UTH & other heat sources to heat up all the substrate & all the other furnishings, so if you take temp. readings too soon, you'll likely get it wrong, & it can be very unsafe for your snake (the heat will keep rising gradually). Also, it's essential to control every heat device with a quality thermostat, & stay vigilant- take temp. readings.