I'm not a big fan of using tanks for the purpose of keeping snakes...when you have a chance, try to purchase a cage such as a neodesha, vision, Habitat Systems, Crescent Moon Creations just to name a few; these cages are designed to not only meet the needs of the snake, but they are also designed to be secure.
With that being said, I feel that your temps are a litte too cool. I would recommend keeping your cool end temps at 80-82 deg F 24 hours a day (or certainly no lower than 78 at any given time). Your basking area should be at around 90-94 deg F. so you are pretty close there (I tend to keep temps at these levels on a fairly constant basis with only a slight night drop). You may have to include a ceramic heating element that can be left on continuously, will increase ambient air temps and boost the basking site temps (that's just one of many options you have). I personally use Pro Product Radiant Heat Panels to heat all of my various herps (monitors, venomous snakes, burms, boas, crocs, just about everything).
Hope this helps. Rob Carmichael