You absolutely should not have a heat rock. The heat rock gets too hot, hot enough to burn your snake. Snakes also do not need light of any sort, and an under tank heater (UTH) that supplies belly heat is the preferred method of heating the warm side of the enclosure to 85F. The light bulb is just going to be a hassle. I would not get it. Then you need a thermostat to regulate the UTH. If you Froogle ZooMed 500R you can find a basic one, or Big Apple Herp sells a nicer one for $32, the BAH-1000. Then you need a reliable thermometer to read the temp on the glass right above the UTH- not on the substrate. I recommend the dual-probe Big Apple Deluxe Thermometer for $25 (I liked the first one I bought so much I just bought three more!) which will tell you the warm and cool temps. It's way easier to just buy the right stuff to begin with and _know_ that your temps are right and your UTH isn't cooking your snake.
With a 40 gallon tank, you should provide multiple hides and branches and vines so your snake isn't afraid to come out into the wide open spaces.
Nanci