Actually Jenn, it's really better to allow the garters to cool off at night. I'd put all the heaters on a timer, so that they all turn off at night. Yeh, your temps. do sound a bit low, and especially if your snake is constantly sitting on the heated side of the tank.....he's telling you he's cold. I usually try to have my "hot" temps. in the mid 80's or so, with the average tank temp. in the mid 70's or so. Also, you're going to want to make sure that your little guy has hiding places on both the cool side and warm side of the tank, so ha can choose where he feels most comfortable. Good luck with him!
Roy