A 40 gallon tank? Really? do you know of any snakes that go into something smaller? I can't really go much above a twenty gallon tank, though I can still try.
Why?
Have you looked at thermostats? That's where your money should go it you're using belly heat.
KMG
0.1 BP 1.1 Blood Python 1.0 Brazilian Rainbow Boa 1.0 Aru Green Tree Python
0.1 Emerald Tree Boa 0.1 Dumeril Boa 0.1 Carpet Python 0.1 Central American Boa
0.1 Brooks Kingsnake 0.1 Speckled Kingsnake 1.0 Western Hognose
0.1 Blonde Madagascar Hognose 1.0 Columbian Boa